As I said there will always be exceptions but overall here in the UK the cigarette and alcohol laws work pretty well, But that is mainly because it is not only the establishment that can get fined, it is also the individual assistant that sold the certificated product, and the company is under no liability to pay if for the employee.
Now most of the problems were employee thinking, 'well if we get caught its not up to me to pay' But with this law, they are more likely to question a border line case than just accept it, because it could be their fine.
And the same goes for a parent buying the product. 99% of the time they are not asked and most I would say are not informed of the explicit nature of some of the
games, but If they new about a law that they could be prosectuted for if they purchased a certificated game for a minor, it may tip the scales and if nothing else, make them aware of it.
But as I said there are always those assistants who dont care, or those parents that think its easier if I get it for them, No law is 100%.