In my experience, it's not the age at which you start playing these games, it's how you learn to play them. For example, I started playing M-rated video games 15 years ago...at 4 years old. I never once became the person who screams incessantly into my headset and constantly demeans the game and other players.
In all honesty, I don't think the game itself necessarily brings out these unsavory qualities in others, but rather with how they were raised along with the people they hang out with. I know many younger kids who love to swear and walk all over authority, and those are the ones who you'll find in Halo 3 ruining the experience for everyone. However, there are still respectful kids out there; 9 year olds who not only respect the game, but respect their teammates. It always warms my heart to find myself playing Left 4 Dead with a younger kid who, while he doesn't always get the right idea, he never whines or swears.
I no longer hold much validity to the notion that "Well, they're too young to play these games anyway," because I simply don't believe that's true (and I feel like a hypocrite). I believe if these kids had better parenting and were really taught the difference between right and wrong; fantasy and reality, and most importantly, respectful and disrespectful, everyone would have a much better experience.