Like the other starting pokemon, Bulbasaur are extinct in the wild; they are the only one of the Kanto starters which was hunted to extinction. It is feared that, as only a select few skillful breeders have managed to induce Bulbasaur to mate, and as the population has dropped to dangerously low levels, they will join the ranks of Aerodactyl and Omanyte within the century.
This is primarily due to a legend which states that if a Bulbasaur's bulb is removed, the Bulbasaur will die but the bulb will continue to bloom and in a decade's time produce a potion which can halt aging for yet another decade. This legend has been debunked by the scientific community, and only the most traditional, superstitious folk and the most learned researchers believe it today.
Professor Oak of Pallet Town is far older than anyone knows.