What an an americentric question. In the UK it isn't an issue, it's the law. If it says 18 on the movie poster you have to be 18 to get in. You could try and bluff your way in of course, but they're used to that.
And I'd say that's a good thing. In the home parents have more responsibility, with TV and DVDs and I think parents can make sensible judgements as when it is right to let children see 12s or 15s. But even then you still can't sell those DVDs to children, parents have got to buy them for children or already have them on their shelf. Mostly the system works, kids see age appropriate movies with a couple of years of leeway depending on what the parents think or whether the kids have an older brother willing to buy it for them.
And I'd say that's a good thing. In the home parents have more responsibility, with TV and DVDs and I think parents can make sensible judgements as when it is right to let children see 12s or 15s. But even then you still can't sell those DVDs to children, parents have got to buy them for children or already have them on their shelf. Mostly the system works, kids see age appropriate movies with a couple of years of leeway depending on what the parents think or whether the kids have an older brother willing to buy it for them.