I'm 16, and I've been playing M rated games since I was 10 or 11. Teen rated games since I was 7ish. I personally feel that not age, but maturity level should dictate if a child/teen gets to play a video game that has violence in it.
I wouldn't let my little cousin who's 12 play modern warfare, because he honestly acts like he's 5 still. He goes out and reenacts everything he sees in video games (Harry Potter taught me that).
Ratings are just another baby sitting mechanism for parents who don't have time for their own children, IMO. If your son is mature (i.e., he realizes that it's a game, he understands that he shouldn't go and try to play modern warfare in the backyard with his friends, he won't get nightmares from scenes in the game, etc.) then I would get it for him. Also, I would let him play online under most circumstances, but that can be iffy for some parents, which I understand.
I got the first Modern Warfare when I was 13 and for me, it was just another video game. It didn't bother me in anyway. I've also been playing GTA for a few years, which isn't as evil of a game as everyone makes it out to be.
-Don