Ideally, maturity, but how is the government supposed to police all the kids who shouldn't be playing these games? The purpose of age ratings isn't to target all of the kids under that age limit, but to reduce the chances that a person who isn't ready for this kind of thing getting hold of the game. Thats how it works in theory anyway. So, if we're going to have age ratings at all, they'll have to be based on age and not maturity. At least until the evil overlords take over and make us all wear mind probes so they can tell how mature we are and sell games according to that measure. (as well as banning their political opponents from watching PG rated shows for the evulz.)
Speaking frankly, I've never seen or heard of a case where a kid has been mortally scarred by a video game anyway. A few cases of misapplied nighmare fuel, but nothing life-warping. Well, except for that kid who shot his parents over halo, but come on, are you going to base your view of the entire gaming community on the actions of one (rather far on in his childhood) nutjob?
Films and TV are far more scarring at times in terms of high octane nightmare fuel and other mind-warping stuff, and just about anyone can access any TV show, regardless of age. Video games are fairly innocent compared to some of the mind screws and blood baths I've seen in the horror genres freely available on your TV set!
But I'm being overcritical. Like I said, you're never going to get a perfect solution. At the moment, we have the age system, not because it is the complete solution to the problem, but because it is the best thing our system can come up with. If you have any means of improving this, please email your local MP and tell him, he'd be thrilled for the chance to advance his career.
I'd say maturity, optimally, but at the moment the only power the law can practically put in place is banning by age.
EDIT: Again, until the maturity probe mark II comes out.
