I don't think it's a matter of a specific age. It more depends on the person's ability to handle the stuff in the game properly. If they're "mature" for their age, and aren't whiny little kids, and they're used to M rated games, then sure, let 'em play.
If they're not used to M rated games, or they can't handle those things properly, or they're just immature about it(more so than most GTA players)..then they should probably stay away from it for now.
I played GTA3 before I was 18, and I was fine with it.
The way I see it, if the kid is too young to understand, that you can do things in a game that you shouldn't do in real life, he shouldn't be playing it. Like, I've seen things on the news about kids stealing keys to drive their parents car, or recently, a kid got on a plane by himself claiming he thought he was playing GTA. If they can't understand not to do that, then don't let the kids play. And if the kid does do that, and people blame the game..let me say: It's the parents fault, not the game. The game clearly says Rated M, and why it's rated M. If the parents are stupid enough to give it to their kids, then it's their fault. That's like showing an 8 year old a scary bloody movie, and blaming the movie when the kid has nightmares.
As far as online goes though, if your kid acts like a kid, he probably shouldn't be on. Simple as that. And if you insist on letting him on, don't give him a mic please. The people that are actually in the right age group for rated M games, don't wanna hear your high-pitched, whiny little kid complain when he dies 40 times, or if someone steals his car.