My one experience with a seriously underage gamer in the group:
Several years ago (I was running AD&D 2e because it was what was current, wow, that makes me feel old) I was running a game. It was composed of people from the gaming club on campus, so people generally from 18 to the mid 20's (I was about 20 at the time).
One person in the group, my best friend actually, wanted to bring his little sister to the game, who was 12 at the time. She was going to be in town with family for the weekend, and would be spending all Saturday (when our game was) with him, and she wanted to try "real" D&D with a whole party. He'd been running one-on-one sessions with her for about a year so she knew the rules and what D&D was, but had been wanting to play the real thing. I had also met his little sister before, as she visited every few months and we'd been introduced and knew each other in passing already (and had otherwise been introduced to most of the gaming group she would be playing with).
I was a little wary of gaming with a 12 year old, but it was only for one session, and it was her big brother (13 years her senior, speaking of teachers, actually he was in the Masters in Education program getting his Master's and Teaching Certification at the time).
The session itself went fine, and no negative repercussions ensued (actually, her dad and I got to know each other in later years, and I apparently made a decent impression on him), but some other members of the group questioned my judgment in it. If she hadn't been with an adult-age sibling or parent there there was no way I would have allowed it.