1988. I was 5. We had just moved to Toronto, and my dad was pretty desperate for a group (he was one of those 1974 people... had eight brothers, and any game that required the group to work together was something the mother encouraged, so they were kind of "pushed" towards D&D). Anyways, he decided to help me roll up a character, an elven ranger I based entirely off Link from Legend of Zelda... I named him "Nelf" (which stood, by the way, for "neutral elf"... why a name has to "stand" for something, I'll never know).
Anyways, the two of us ran through the solo adventure designer found in the DMG, with my elven ranger and his human ranger pretty much taking out any monster we found. It was fun, even if all it ever consisted of was dice rolling.
My first real "Campaign" was in grade six, which would've been in... I dunno, 1994? 95? Isle of Dread, baby.
Edit: Ha. I should've guessed Gygax would do his best to ruin the poll. Should someone put in a "1972" option, to, y'know, keep the whole thing accurate?