Well i hope the maturity level is in tone with the types of games that are being played. ESRB
i'd say maturity level on PSN is 65/35 maturity, but thats only when you can hear them. But im pretty sure there is ton more kids with mics on xbl.
IMO, it really depends on what game you're playing, how serious the match is, and who you're playing with.
If you're in a public room with randoms, try to be mature and respectful.
In a (serious) private or competitive room (clan matches and the like), maturity is expected, as you need to be on top of your game. Good sportsmanship is also always a good thing, and being a bad sport can get you and your team a bad rep.
In a room with a bunch of friends (private/public, ranked/unranked) that's there just to goof off...well, you know your friends best and what's acceptable and not.
As for the game...if its a "silly" game where goofing off doesn't hurt anyone, people usually don't care. If its a more serious game...be more mature.
The best way to **bleep** these guys off is to team kill them where available.
But that make you as 'immature', as they are? Isn't there a way to remove 'offensive' players from a game?