The absolute youngest I've met was 12. A resto druid in Vanilla that could actually heal in such dungeons as 5 man LBRS and UD Strat. Yeah, he was good.
The conversations in teamspeak were just beyond hilarious, as his squeaky prepubescent voice incited raucous laughter ever time he talked.
One conversation is still pretty fresh in my memory from back then. Him, myself and a new recruit to our guild were in TS, and i had no clue the kid was in TS. I remarked being hung like a tic tac, I had no ego, and he remarked, and I quote, "Hung like a tic tac? How can a tic tac be hung?" Being as I didn't know he was there, and the high pitch of his voice, I laughed so hard and kept laughing so hard I fell out of my chair.