Me, my siblings and my friends have all been playing 18 rated games since we were 7/8, and it hasn't had a detrimental effect on any of us. I'm against censorship of all kinds from a Governmental level, I also don't think that parents should have the right to control what I watch/read/listen/play beyond a certain age (though I'm not sure what that age is).
My mum, for example, has always had a big problem for the GTA games because it uses prostitution, however, I do not think that she should have the right to restrict me from playing those games just because she has a problem with them (though, I don't really like GTA games anyways, although I have been looking at GTA4 with some interest, recently)