wutI don't think kids should be allowed to play rated M games. Not only they are too disturbing for their age group, but they're turning into trashy kids.
Did you know that most of the people who cuss online, talk about sex and nudity online, and those who say "go to hell" a lot online are actually immature people under the age of 13? It's true.
I've been playing M-rated games for over a decade and I'm not the kind of person to go yelling outside at 3AM in the morning or blasting my subwoofers at such a volume that people's houses literally shake. Those are actually the people who play little to no video games, so I'm not sure what kind of people you're running into.
And from personal experience I can say that playing rated M games can help in a person's development. At least it did for me. I decided to play a bunch of horror games (rated 16-18 here, I was 13 when I started), and they helped a lot with my nerves. I was really jumpy before that, scared of a bunch of things without any reason. Horror games made me face that and mature.
Things like violence in video games is also fine. It's just a fact of life. If people want to shelter their kids that much they should keep them at home, because there's enough violence, strong language, etc. just in a school environment. Whether or not their kids are playing games that are rated for ages higher than theirs is not going to help.