Basically, I'd decide.
A lot of the ratings are or seem arbitrary. For example, I imagine with no blood splatter and no dismemberment just because Australia's old man government doesn't want that, L4D2 still got a very high rating. Being perfectly honest, removing those things, the game should actually rate very low compared to many war related games. If a game is rated high for blood or gore, I'd look at the overall message of the game. GTA, obviously not, since it rewards you for more death and carnage. A similar nix on most war games. However, though few, a game that actually teaches the value of life, and shows how horrible death is on a personal level, that I'd be ok with. Especially if it doesn't then encourage vengeance (So SR2, while having some heart-rending moments, would not qualify.)
If a game is rated high for cursing, I wouldn't care as much. Any kid I raise needs to understand there are some things you don't say in some company that's ok in other company, and definitely needs to understand that saying the wrong thing could get you into trouble, if not a full-on fist fight.
Fighting games, a definite not-until-they-are-at-least-12. However, I wouldn't mind sending them to fighting classes as early as 5, especially since that teaches real consequences to fighting.
Alcohol... I want my kids growing up with a respect for alcohol, rather than a fear. So especially something like Fable, where it shows your character vomiting when they drink too much, yeah, I'd be fine with that.
Sex, blatant sex, I'd say 14 is probably alright (this really depends on the age when they will be ok with the realization that Mom and Dad still have sex, and likely, so do my parents.) Adult themes could be even younger. Hell, I'd probably let my kids play Catherine the minute they're old enough to understand the words. Again, respect is more important than fear.
Realistically, any violent games in my house would probably be there because I'm playing them, so I should have a fairly good estimate of what my kids are getting into. Most of all, I'd really appreciate being able to play with them, so I can educate and supervise while they play, and so I can get a little bonding time in.
Ah, the one possible exception to these rules (all of the above) would be (real, not shoot-em-up) survival horror. They're like the scary stories of our age, and really depend on a kid-by-kid basis there.