Sometimes barriers to entry are nice. Case in point: Guild Wars seems to have a lower average player age than World of Warcraft (yes, it's possible). I'm certain that this is because there's no monthly fee: parents who would not be willing to buy their child a game that costs them an extra $15 until that kid gets bored would be much happer to not have to pay a monthly fee.
Maybe, maybe a solution would be some kind of adults-only mode, like the ability to block out all chat from younger players. This certainly isn't a surefire fix, but it might be nice. Acrophobia, the only online game I played with any regularity before World of Warcraft, had public rooms and adults-only rooms; I always picked the adults-only rooms. The topics of conversation were just usually less stupid in the adults-only rooms, though you got your fair share of younger-seeming people who just wanted to be able to say "tits" without it being bleeped out. There was no actual check that you were 18; you just picked a button at the start. Maybe 18+ servers in Guild Wars and World of Warcraft would be nice. But then again, it could very well never work.