Well, their infernal squeaking over chat really makes me wish they didn't play any M-rated games.
However, it really comes down to the child, the game, and the rules and restrictions the parents have. I do think parents should be well-aware of what their child is getting into, and the child should at least be partially monitored for any inappropriate behavior (or exposure to inappropriate behavior), but, on the whole, I think this should be taken as a case-by-case basis, not on a universal level. I mean, I know parents who let their younger children play so long as they (the parent) is playing with them or monitoring them, and those kids are more well-behaved than half the adults playing the game.