I'm all for protecting children if there is genuinely a danger, but methinks this is a rubbish way of going about it. Sure there is an age where children shouldn't be able to access content currently rated at 18+, but I'm not sure that this is it. And I'm not sure that this would solve it even if it was enforceable.
We would be far better off if parents sat down with their children and explained that there is violence / sex / whatever portrayed within this game. It's not representative of real life. Treat it as a fantasy and a way to blow off the stresses of your real life.
Wouldn't that be far more sensible than an unenforceable legislation? Even if shops abide to it and people don't buy from abroad and adults don't buy the games on behalf of children they'll still play it round a mates house who has an older brother who got it.
It's the same with all the f**king stupid legislation they keep trying to introduce to block porn. Massively expensive, never works, and doesn't achieve what they set out for it to achieve.