You Grew Up Playing Shoot'em-Up Games. Why Can't Your Kids? (http://www.wired.com/gaming/virtualworlds/commentary/games/2007/04/gamesfrontiers_0409)
^ I like the "LEGO test" idea.
The LEGO test is an interesting rule-of-thumb. I have to second the notion that if they're not old enough to play the game, they're not old enough to watch the game, either. This means for the oldest in a family of four, say, that he won't get to play the gory games all that much as the little brothers and sisters are around most of the time.