There is either one hell of a lot of kids playing, or one hell of a lot of adults with a lot of growing up to do!
Same goes for this forum really too
This is how I feel with all of gaming in general. Especially when it comes to people like in the OP.
I find that there are FAR more adults that act annoying, childish, and downright ignorant than most of the children I find in online games.
Kids aren't too bad. Apart from their annoyingly high voice, the majority of them aren't even bad. Surely not as bad as some of these "adults" are.
As for the M rating, well, that is a recommendation, not a requirement.
It is recommended to supervise a child while playing, for said depictions of violence or whatever that it lists in the rating.
I find that it's mostly pointless. Video games really don't have a negative effect on a child unless he is mentally deficient in some way to begin with.
Though I think supervision is necessary, not because of the game rating, but just to keep a kid from getting addicted to a game.
You know, making him go outside for a while instead of staring at a TV.