My dad needed a player.
I was five, we had just moved to Toronto, and my dad had all these 1e books (he had started in '78 or so, because he had eight brothers, and the family didn't have much money. But if you buy one game that'll keep 'em all occupied....)
So, we made up characters, one each, and ran them through the solo dungeon rules found in the 1e DMG. Something I still love, and was so glad Paizo reprinted them in the dragon compendium. He would describe to me what it was we were seeing, and he made sure to re-roll things till we got the good treasure or fought the perfect badguy. It was very munchkin, but I was five. I loved it.
My first RL gaming group was much later. I was about ten or so, and I'd been D&Ding with my dad for a few years (though it slowed down when he found some military buddies that also played... but he did take me to the guy's house a few times, to show off the dude's massive mini's collection). Anyways, I met a few friends in fifth grade, and we started playing Basic D&D (the red box collection, though I don't remember this adventure in the DM's set?) I remember running the isle of dread, and the Hidden Shrine of Tamoachan, both from my dad's gaming collection... even though they were for different versions of the game.
From there, we tried out shadowrun (in grade five!), and battletech. By eighth grade, I had something like ten different games on my best. When the M:tG craze hit around then, I was able to play with players, beat them soundly (with a cheaper deck, only because I knew the rules and how these games worked better than they did), and quickly and quietly convert them to a non-D&D RPG.
(If you say "It's dungeons and dragons", people have instant associations. I quickly found that by saying "it's this new game I made up", and then running D&D from memory and a few notes, I could get people in on it, no problem. I even got two "popular" girls turn me down to a D&D game, and then, the next week, play my "new game", and LOVE IT. Funny stuff, junior high...)