It should just be left up to the parents. They can decide what to do. If they think giving gory games to 7 year olds is a good idea, then fine. If they think letting their 16 year olds play Nintendogs only is acceptable, then fine.
It should be the parents responsibility to raise their child. Whether they end up as spoilt stuffs or naive scaredy-cats, then whatever.
If parents are responsible for bringing up their kids in all other places, they should be repsonbile for letting them play games.
My parents have always decided if a game is suitable or not.
Most of the time, they've had no real problem with games.
If it's an 18 and we're too young to buy it, they'll buy it for us if they think we're fine with it.
Happily, I'm 18 this year, so i can buy them whenever I want. And I've been purchasing 18 games online anyway.