Left4Dead (1 and 2) are the only games online during which I spend that much time, and most kids are fairly respectful (more so, if they're aware it's an m-rated game.) So far, my experiences with them have been generally polite and cordial. They're also generally more than happy to pick up cues to improve their game (e.g. don't stray too far ahead or you'll get eaten by Hunters).[footnote]Dunno about the insta-kill Witches in Realism Mode; Everybody seems to be drawn to them like touching wet paint.[/footnote]
Chatters and shouters mostly have been adolescents and adults. Then there's those who pipe music in over the mic, and there's outright griefers who will pipe various sounds through just to disrupt play, but the physical ages of these people are less than clear.
As someone who is quite aware of the issues of minority and identity groups (and who believes in efforts to normalize them in American society), I'm particularly miffed by common use of gay epithets (generally fag or ******, but more often, gay used to describe something negatively). But that is plenty common amongst adults as it is amongst kids.
I would also argue that dangers to our children are overstated in US media, and kids are not only resiliant to events in the living world, but are able to determine the difference between fantastic situations (e.g. video games) and real-world ones (e.g. domestic violence), ergo, don't need to be shielded from M-rated games.[footnote]Or, for that matter, some of the tamer AO rated ones.[/footnote]
238U.