It's not on Wikipedia (yet), but Homer Simpson's birthdate is May 12 1956, according to Simpsons Season 4, episode 16, called "Duffless."
Now you know.
UPDATE/This Just In:
In the same episode, Homer sings about his first beer, and using a fake ID to buy it. The name on the ID was Brian McGee and Brian's birthdate was August 2 1948, boosting Homer's purported age almost eight years.