First of all, if I ever have kids, they'll be playing old NES games to understand how good they have it (probably Megamans, where I'd make it a contest between us to see how much further we could get in killing the boss since I know from experience that not many people actually can beat a level and then the boss in those games). Then I'd probably let them have more T rated games around ages 9 or 10, and they'd probably be the barely teen stuff. However, I would allow M for late 15-16 years old.
There would be the exceptions, like Halo (offline) during T range, but I would definitely not allow them to play a game I don't know about.
I've been noticing the trend of games being more M rated as time goes on, which is somewhat depressing given that you still can tell a story with an E rated game (look at Kingdom Hearts). Instead kids get liscened games that they soon learn are crap or sports games.