Someone else here said T games whenever, and M & online games in high school. I think that's pretty reasonable, but I would still play them myself beforehand, or at least watch trailers and read reviews, while they're not in high school yet.
Personally I always resented my parents for not letting me play T games until I was 13, and M games until I was 17 (although I bought some on my own at 15 and 16 when I didn't get ID'd at the store). Not that I really cared about all the violence or whatever, it's just that all my friends were playing them, and talked about them at school, and I just felt pretty left out of discussions (not just about games, but TV and movies too--I wasn't allowed to watch the Simpsons, Jaws, Godzilla or the Excorcist until I was 17, although I did anyway), and my mom never let me go over to their houses if she knew we would be playing games above our age rating.
I would never want my kid to be excluded like that, and my parents were definitely over-protective. I lived a quite sheltered childhood, and didn't experience a lot of stuff my friends did or saw, and as a result I still am quite isolated as an adult. I'm not blaming my parents for my isolation now directly, just saying that I was unable to join social groups at a young age where I could have developed more social skills early on.
As a side note, my parents (my mom, at least) were devout Christians and I went to church every sunday until I was 15, when I threatened to run the hell away if they made me go anymore. Best thing I ever did.