8 years old for me. My father had always done historical miniature gaming, and there was a small little "convention" at my older brother's high school. At first, I just watched my father and brother play, and both seemed interesting. I later bugged my parents to buy the the core books, and I remember being excited when Deities and Demigods came out.
I remember it was just 2 other friends from school and we only played a handful of sessions. I didn't get seriously back into it until I was 10 though. Perhaps unusually, my run with AD&D was not long lived at all. Once I got back into RPG's when i was 10, the next game I played was a handful of sessions of Top Secret, Villains and Vigilantes, and Champions. Then a lot of one shots (I remember playing some Runequest, Boot Hill, and Gangbusters). I don't think we settled down to a long term campaign until Twilight 2000 came out when I was 12.