I was first exposed in pre-school, age 5 (back in 1975). My best friend's older brother had played D&D, and his older brother was cool. So my pre-school imagination games were all "playing D&D" - but of course, that wasn't actually playing the game.
I tried playing in school in 2nd and 3rd grade, but it was still more like "Let's Pretend" then really playing by the rules. I got the Basic boxed set in 5th grade, and that's when I started to play the real game.
My son was exposed from even earlier since his mom and dad played a lot. I'd say he got into real play similarly around age 10. However, he never had a regular group of his own until college, because none of his local friends played.