I work in a computer games shop in the UK, and over here we have very strict regulations on the sale of games.
The British Board of Film Classification (BBFC) ratings are law, and any sales assistant caught selling an age rated games to someone under that age can face a £5000 fine, loss of employment or even prison time.
I know that it may seem unconstitutional in the States, and dont get me wrong I think your constitution is a fantastic document, but im all for age restriction of violent games. I dont think 10 yeard old kids should be playing games like GTA, or other excessively violent ones.
We also have the PEGI system, which is a general European rating system, which is not legally binding, but its only used for games rated 16 or less.
Just my 2 pence