A/N: Written (late) for the Day 13: AU prompt of the #20yearsatsea challenge on tumblr.

In this AU, Luffy is born thirteen years later than he was in canon. Also, Sabo never tried to sail out early on his own, so he didn't have his amnesia-inducing accident and ended up growing up with Ace on Mt. Corvo.

Unbeta'ed, so apologies in advance for any mistakes!

Garp thought it was a brilliant plan, even if everyone else - Sengoku, Tsuru, Bogard, and even Makino and Woop Slap from down in the village - thought otherwise.

"What the hell is that ?" Ace hissed, eyes narrowed in both suspicion and contempt at the little bundle in Garp's arms.

Ordinarily Garp would have rewarded the sixteen-year-old's tone with a righteous Fist of Love, but he was in a good mood, so instead he let out his usual booming laugh. Then, grinning, he bent down and moved the little bundle of blankets so both Ace and Sabo - who'd been shifting nervously at Ace's side - could get a better look.

Ace wrinkled his nose, looking confused, while Sabo blinked in bemusement. Then Sabo's eyes went wide. "Garp-san -" Garp's grin faltered; hadn't he told the boy to call him Gramps? "- is that… is that a baby ?"

Garp beamed. "Sure is!" he exclaimed, before promptly dumping said baby into Ace's arms. Ace yelped and jerked away, almost dropping him, but Sabo lunged and caught him just in time - then ruined the act of heroism by immediately trying to shove the baby back into Ace's arms, expression panicked. "Meet Monkey D. Luffy - your new baby brother!"

Dadan, who'd been warily watching the exchange from the doorway of her shack, started choking.

"Oh, no, not another one!" she cried, shaking her hand back in forth in front of her face. "No, no, it was bad enough with these two demons -" She pointed to Ace and Sabo, who were now playing an awkward game baby hot potato, identical looks of alarm on their faces. "- and I'm not taking care of another one! I won't! I refuse, you ask too much -"

"You won't have to," Garp interrupted, a pinky stuck casually up his nose. "Ace and Sabo will take care of him."

There was a moment of stunned silence, in which Sabo lost the game of baby shuffle, and everyone stared, slack-jawed, at Garp. Then -

"WHAT ?"

"You can't be serious!" Ace sputtered, pointing at Garp angrily.

"They'll kill him," Dadan blurted, looking almost more alarmed than she had when she'd thought she was going to have to take care of the kid.

"We don't know the first thing about babies," Sabo protested, his panic rising even more when little Luffy - roused from his nap by all the commotion - started fussing in his arms.

Ace whipped his head around to stare at the baby, his horror apparently mounting at the realization that it also made noise.

Garp let out another loud laugh, then bent down to give his baby grandson's tummy a poke. "You be good for your big brothers, now, Luffy," he crooned, which, for some reason, made Ace and Sabo start freaking out even more.

"You're insane!" Ace shrieked, eyes darting between Luffy and Garp, and looking even more disturbed when Garp's cooing actually worked and settled the boy. Garp ignored him, choosing instead to watch in amusement as Sabo did an excellent impression of a cornered animal and stared down at Luffy, who blinked back up at him in sleepy curiosity.

"Wait, Garp - did you say Monkey D. Luffy?" Dadan asked in a strangled tone, catching everyone's attention and cutting off the rest of Ace's rant. "As in… as in your biological grandson? Dra- I mean, that man's child…?"

"Eh? Oh, yeah, of course. He's my son Dragon's boy," Garp said, picking his nose again. "Don't worry, that doesn't mean I love him any more than you," he added as an aside, nodding to Ace and Sabo. Wouldn't want them thinking he was picking favorites or anything.

Dadan went white as a sheet, while Ace turned a lovely shade of puce. "THAT'S NOT THE-"

"Where are his parents?" Sabo interjected, before Ace could get any louder. "I mean… if he's your son's son, then why isn't he…?"

Garp grunted. "They didn't want him," he said, which wasn't entirely a lie. Up until presenting Garp with the news that he was now a grandpa to his first grandson by blood, Dragon had always claimed he didn't want children. "And besides, his father is the most wanted man in the world - do you really think he'd be safe with him? If anyone found out whose son he was, they'd take it out on him."

Sabo glanced sharply at Ace, whose mouth had snapped shut. Dadan made a pitiful noise behind them, no doubt at the prospect of being caught harboring not one, but two famous criminals' sons.

Ace gritted his teeth and glared at the ground for second, then went back to scowling at Garp. "I'm still not taking care of him," he growled, fists clenched.

Annoyed, Garp scowled back, and was reconsidering his temporary ban on Fists of Love when Sabo made a soft sound that caught both his and Ace's attention.

Hoping Sabo hadn't actually dropped Luffy, Garp turned, and was greeted with a sight that forced him to hold back a bark of laughter: Sabo looking down at Luffy with an expression of absolute wonder while the baby happily sucked on one of Sabo's fingers.

Well, that was one down.

"See? He likes you," Garp said, grinning, while Ace gave Sabo a look of utter betrayal.

"Sabo, get your hand out of his mouth, that's disgusting -"

"Uh-huh," Sabo said absently, completely ignoring Ace. Then, without ever taking his eyes off of Luffy, he asked, "What did you say his name was, again?"

Garp beamed. "Luffy," he said, reaching out and patting him on the head. "His name is Luffy."

Dadan's instincts to protect defenseless babies apparently overran her dislike of Garp's devil spawn, because she made Ace and Sabo stay with her and the bandits instead of returning to their treehouse that night.

"Until I'm sure you can take care of him without killing him," she said, while Ace sulked angrily against the wall.

"Why do you care?" Ace snapped, watching in disgust as Magra, off all people, walked Sabo through changing the little drool-monster's diaper.

"Because I'll be the one Garp takes it out on if he comes back and finds out you idiots let him get eaten by a bear," she snapped back, a vein throbbing in her forehead.

Ace scowled and crossed his arms. "What do you guys even know about taking care of babies, anyway?" he challenged.

Dadan stilled for a second, an odd look on her face. "We managed with you, didn't we?"

Ace blinked back at her. Oh. Yeah.

For some reason, the realization made him uncomfortable, which was why he was actually glad when Dadan ruined any maternal images he could have mustered of her by scoffing and adding, "At least this one's cute. You were a pretty ugly baby."

"You were. We have pictures," Dogra supplied helpfully, while Sabo laughed and both Ace and Dadan sputtered.

Thankfully, the arrival of another bandit with a baby bottle spared Ace from analyzing the implications of the group keeping his baby pictures.

Once Dadan had showed Sabo how to hold the baby while he fed him, they all sat back and watched as the baby sucked down the bottle's contents at an alarming speed.

"Is that normal?" Ace asked, in spite of himself. Nobody answered him, but Dadan looked pained again.

Once the bottle was empty, the brat's eyes started to close, which only furthered Ace's annoyance with him. If this kid was just going to eat, sleep, and poop, what was he good for?

Apparently this sentiment wasn't shared by Sabo, who got weird, sappy look on his face, and settled the brat more firmly against his chest.

"How could anyone just… not want him?" he said softly, running his hand up and down the baby's back.

"I don't want him," Ace pointed out, but, continuing with the theme of the day, everyone ignored him.

Four weeks later, and Ace still didn't get why Sabo was so attached to the brat - or Luffy, as Sabo kept insisting they call him.

Ace remained stalwart in his refusal to take care of the kid, and Sabo had mostly stopped bugging him about it - especially after the third time he'd tried to get Ace to hold him, and Ace had dropped him. (Ace still didn't see what the big deal was; the kid was fine, and hadn't even cried all that much. Certainly not enough to warrant the violent clubbing Ace had gotten from Sabo's pipe.)

Still, Sabo couldn't be with the br- Luffy - every second of every day, and Ace knew it was only a matter of time before Sabo tried to force Ace to help anyway.

That being said, Ace didn't think that time would come in the form of a screaming baby in the wee hours of the morning.

"Sabo, shut that kid up," Ace groaned, rolling over and covering his ears with his pillow. When he received no response and the crying became louder, Ace picked up the pillow and tossed it in Sabo's direction. "Hey, come on - Sabo, your baby is crying!"

Sabo didn't move, though, or say anything, so after another minute had passed and the din became unbearable, Ace kicked off his blanket and sat up, feeling murderous. "Oi, Sabo!"

Intent on giving the blond a good kick, he shuffled over to Sabo's bedroll - only to come up short when he found it empty. "Sabo?"

He frowned down at the bedroll, then gave the treehouse a once-over before finally settling his gaze on the door. Sabo's pipe wasn't leaning against the wall, like it usually was, and his hat was gone as well, which meant….

"No," Ace muttered, feeling a sudden wash of horror. "No no no, you did not leave me alone with..."

A particularly ear-piercing wail split the air before he could finish his sentence, making Ace swear and turn towards the source of all the noise. He was going to strangle Sabo when he came back.

Knowing it was the only way he was going to get any peace and quiet, Ace started towards the baby's bed, then stopped and swore again, his nerve failing. What the hell was he even going to do? He definitely wasn't changing the kid's diaper, if that was his problem (not that he knew how, anyway), and he had no idea where Sabo kept his bottles if he was hungry. He'd heard Sabo singing softly to the kid a few times when he just needed soothing, but Ace would be damned if he did anything of that sort.

He could just leave. Hunt Sabo down and make him come back and deal with this, which would have the added benefit of getting away from the noise, but...

But Sabo would probably kill him, and besides, as much as he claimed otherwise, even Ace wasn't heartless enough to leave a crying baby all alone (which, come to think of it, was probably what Sabo had been counting on, the bastard).

"Alright, alright, I'm coming," Ace fumed, and stomped over to the shrieking menace. As if sensing his presence, the baby's cries got quieter as Ace approached him, though not nearly enough for Ace's abused ears, in his opinion. To Ace's dismay, however, he started right back up again once Ace was beside him - this time with big, heartbreaking sobs that shook his whole tiny body, and made Ace feel rotten despite his dislike of the kid.

"Hey, don't… don't do that," Ace grumbled, staring into the little basket Sabo had fashioned into the baby's bed. Ace was surprised when the kid - Luffy - actually listened, even it was just to downgrade to sniffling pitifully. Anything was better than the screaming.

Relieved but unnerved, Ace stuck his hand on the side of the basket and gave it a little rock, something he'd seen Sabo do a few times. "Uh, hey, yeah… see? Your… Sabo, will be back soon -" Or at least he'd better be. "- and then you can, uh… do whatever it is you do, and… why are you staring at me like that?" Ace asked, bemused, as he reached down to move the blankets away from the boy's face.

Luffy gave another pitiful sniffle, his small, tear-stained face red from exertion. Then, before Ace could move away, he wrapped his whole, tiny hand around Ace's finger.

Ace blinked down at him stupidly, mouth falling open as wide, innocent, impossibly liquid eyes blinked back at him.

"Oh," he breathed, a strange, warm feeling exploding in his chest.

There was a chuckle from the doorway.

"Yeah," Sabo said quietly when Ace turned his head, a dopey grin on his face. "He does that to you."

Maybe, Ace decided later as he fed Luffy his bottle, Luffy's hand still wrapped firmly around one of Ace's fingers. Maybe a baby brother isn't so bad after all .

"Ah, ah, ah!" Luffy cried, whole body wiggling as his arms reached for Ace - much to the chagrin of Sabo, who was holding him.

Ace smirked. "You hear that? He's trying to say my name," he declared smugly, picking Luffy up out of Sabo's lap and swinging him into the air. "Ace, Ace, Ace - can you say Ace, Luffy?" Ace cooed, making silly faces and swinging Luffy higher with every chant of his name.

The Ace of six months ago probably would have spontaneously combusted if he'd seen himself doing something so stupid, but present-Ace didn't care; that Ace hadn't had a sweet, adorable baby brother who had a smile like sunshine and worshiped the ground he walked on.

Sabo rolled his eyes. "It's just baby babble, it doesn't mean anything," he insisted sullenly, his arms crossed.

"Ah, ah, ah!" Luffy cried out, mimicking Ace and shouting every time Ace swung him into the air.

Ace lowered Luffy and grinned triumphantly at Sabo, who glared murderously back.

"It's one fucking syllable, of course he's going to have an easier time-"

"AH!" Luffy shrieked, and smacked Ace in the face with a flailing hand. Sabo burst into laughter, which in turn made Luffy giggle and make grabby hands towards Sabo.

"Hey, hey, I thought I was your favorite, Luffy. You can't go abandoning your big brother so easily!" Ace protested, while Sabo shot him a smug look and moved to take Luffy back.

Luffy squealed in delight, wiggling even more and whacking both brothers with more flailing limbs. "Bah!" he cried, beaming up at Sabo's face once Sabo finally wrestled him free of Ace. "Bah, bah, bah!"

Both Sabo and Ace froze. Then a slow, elated smile spread over Sabo's face. "Bah," he said, with much more reverence than such a syllable probably warranted. "I'm 'Bah'!"

Ace snorted out a laugh. "Bah - what's that short for, 'bastard'? I think it suits-" He stopped and blinked, eyes narrowing in disbelief. "Are… are you crying?!"

"N-no…"

"You are!"

"Sh-shut up, asshole."

"Ah!"

"He's got you both wrapped around his little finger," Dadan snickered one evening, after Ace and Sabo had decided to grace her with their presence (and three boars) for dinner.

"I don't know what you're talking about," Ace grunted around the hunk of meat in his mouth, while Luffy banged a spoon on the floor and drooled happily in Ace's lap.

"Really?" Dadan snorted, eyes darting over to Sabo, who was alternating between making silly faces at Luffy and glaring moodily at Ace.

There'd been a bit of an argument over who got to hold him, but Ace had won after pointing out that Sabo had been hogging him all morning, and that Ace was better at keeping him from putting strange things in his mouth while they were eating.

"Can I hold him, then?" Dadan asked, one eyebrow raised.

The grip Ace had on Luffy tightened. "No."

"How about me?" Magra - who'd been fawning over Luffy since they'd gotten there - asked hopefully.

"If I don't get to hold him, the rest of you don't, either!" Sabo snapped, a vein throbbing in his temple.

"What happened to not wanting him?" Ace added, scowling at Dadan.

Dadan waved her hand dismissively. "I didn't want him when I thought I was the one who was going to have to raise him. I'm fine with him so long as he's still your problem at the end of the day." She leaned over and wiggled a few fingers in Luffy's face, which elicited a happy exclamation of "Da!" from the boy. "Like I said - he's kinda cute."

"Of course he's cute, he's the cutest baby in the whole world," Sabo interjected, as though they had been disputing it. "I mean, look at him! Those big eyes, that cute face…"

They all stopped to admire Luffy's cuteness. Luffy, as if sensing the scrutiny, gave the room a big, gummy grin.

Sabo actually covered his face, as though shielding himself from the adorableness. Ace didn't know what his own expression looked like, but from the way Dadan was staring at him, he figured it was probably just as stupid.

The days seemed to pass quicker than they had before Luffy came into their lives. Ace's seventeenth birthday came and went, but the significance of it didn't hit Ace until afterward, when Sabo's started creeping up as well.

Because Ace remembered their big plans, the ones they'd been making since they were just kids. How they had both decided to ship out for a life of piracy and adventure as soon as they turned seventeen. Ace still wanted those things, and didn't want to give up his dream, but at the same time…

"Hey, Sabo?" Ace asked one night in early March, just a few weeks shy of Sabo's birthday. "What's going to happen to Luffy when we leave?"

He heard Sabo suck in a sharp breath, then let it out slowly. "You've been thinking about it too, huh?" he said quietly, rolling over so Ace could see his face. Ever since Luffy had learned to crawl, they'd taken to sleeping with their bedrolls together so they could keep Luffy in between them (and wake up if he tried crawling off and over them).

Ace grunted an affirmative and tightened his grip on Luffy, who was tucked into the crook of Ace's arm, sound asleep.

Sabo looked down at him, the corners of his mouth tipping up in what had become an unconscious reflex when it came to looking at Luffy, though the smile dropped just as quickly.

"I don't know," he said, and Ace could tell by his tone that this had been weighing on him just as much as it had Ace. "I… I still want to go out to sea, but at the same time…"

"We can't just leave him here," Ace finished softly, just the thought leaving a phantom ache in his chest. Maybe if Luffy were older, or could fend for himself…

Sabo's brow furrowed, his eyes taking on that determined, calculating glint they sometimes got when he was planning something.

"Maybe," he said finally, face spreading into a wide, sly smile. "Maybe we wouldn't have to."

"We couldn't possibly take him with us," Ace started to protest, only for his words to die in his throat at the look Sabo was giving him. "...could we?"

Sabo's smile widened further. Then he told Ace what he was thinking.

"Gramps would kill us," Ace said once Sabo had finished, sitting up now with Luffy pressed against his shoulder. Still, the thought couldn't squelch the smile threatening his lips, or the growing hope in his chest.

"He'd have to catch us first," Sabo pointed out wryly Then his face turned serious again. "We'd have to do it together, though, instead of leaving separately like we'd originally planned."

Ace smirked. "Wouldn't have it any other way," he said, and didn't miss the way Sabo's shoulders slumped in relief.

Luffy chose that moment to snuffle awake, his head popping off Ace's shoulder long enough for him to blink sleepily. Then he settled back down, head turned and face tucked against Ace's neck.

Ace chuckled and dropped a kiss to his crown. "What do you think, Luffy?" he whispered, his breath ruffling the boy's hair. "Wanna be a pirate?"

"WHAT DO YOU MEAN, THEY'RE GONE?!" Garp bellowed six weeks later, nearly foaming at the mouth in rage.

"It's not like I could have stopped them!" Dadan shouted back in a rare moment of bravery, nursing the lump Garp had already left on her head.

"YOU COULD HAVE TRIED!" Garp howled back, his anger the only thing keeping his panic at bay.

Ace and Sabo could handle themselves - Garp's hellacious training regiment had helped see to that - but Luffy was only a baby! He could barely walk, let alone fight, or even swim - what if he crawled off the boat and fell into the sea? What if the boat sank and Sabo and Ace couldn't get to him in time? What if he got eaten by a Neptunian?!

"And they said they were heading for the Grand Line?" Garp demanded, while horrifying visions of his baby grandson getting eaten by a sea monster danced in his head.

"Th-that's what the letter we found said," Dadan replied warily, backing up well out of Garp's reach.

Garp ran a hand over his face and turned his back on her, swearing to himself. It wasn't really the old bandit's fault, he knew; Ace and Sabo had always been obstinate - Ace alone had stubbornness down to an art - and if they'd really been determined to leave, there wouldn't have been any stopping them. Not unless Garp himself had stood in their way.

(Why hadn't he come just two weeks earlier? Why?)

Damn it, he'd have to tell Dragon, too, when he called for an update on Luffy. His son was never going to let him live this down.

"Garp-san was awfully angry," Makino noted, setting down the three drinks her customers had ordered.

"As he should be," Mayor Woop Slap grumbled. "Who takes a baby out to sea with them, and into a life of piracy, no less?"

"Oh, shut up, you old goat," Ace snapped, grabbing his and Sabo's drinks since Sabo's hands were full with Luffy. "We were only gone for two lousy weeks, and we didn't even go that far. Gramps probably sailed right by the island we were docked at."

Woop Slap scowled, which seemed to amuse Luffy, if his giggle was anything to go by. "Whether or not you actually have yet, you're still planning on it!"

"Yes, but not until Luffy's old enough," Sabo pointed out, accepting his drink from Ace and giving it a sip. "For now, we'll settle for sailing around the East Blue to get a feel for things, and staying off Garp's radar. At least until Luffy gets his sea legs."

"Or legs at all?" Woop Slap grumbled, tapping his glass against the countertop in agitation.

Makino laughed as though Woop Slap had been joking, then walked around the counter to hand Sabo the bottle of milk he'd requested. "I think it all sounds very exciting," she said, giving Ace and Sabo a sunny smile that could rival Luffy's.

"You say that because Garp didn't threaten to murder you," Dadan grumbled from her seat in the corner, still nursing her many lumps. Ace made a mental note to buy her a bottle of Makino's best booze.

Sabo cleared his throat. "In any case, I think it goes without saying that none of you breathe a word of this to Garp," he said, his serious face belied by the babbling baby reaching for his hat.

"Or we really will take off for the Grand Line, baby or no baby," Ace added as emotional blackmail.

There was a slight grumble from the assembled conspirators - mostly Woop Slap - but Luffy's triumphant laughter at finally reaching Sabo's hat drowned most of it out.

A/N:

So... yeah. Go ahead, bring your baby brother on your journey to become a famous pirate. What could possibly go wrong? XD

I'm marking this as complete for now, but I'm probably going to come back to it because I have too many Ace and Sabo and baby Luffy on the high seas feels.