12, my friends mum was DMing and writing and we played through the school holidays. 7 of us... poor woman! I’m 41 now. I know I was 12 because the Tome of Magic had just come out, and it the only book I owned.
1981, maybe 1982 at the very latest. I was 6 or 7 when my fathers friend introduced me to D&D and I sat in on a game with them. I'll be 46 in June. Pretty sure I owned a D&D boxed set before an Atari 2600.