Honestly, it depends from person to person. I started playing M rated games when I was 9. But they didn't have an effect on me (I never thought about actually killing people.) So yeah.
I don't think that all games that are posted with rated M Deserve the rating. Like a Halo game in movie rating standards it would probably be PG-13 meanwhile a game like Gears of War would be rated R because of the sever level of Gore. He plays halo and I'm ok with that but I don't think he should play gears. Crackdowns ok but not saints row you get what I'm saying?