There is no law in the US restricting sales of games based on the content, aside from the laws that prevents selling adult movies/magazines to minors. Since games that would violate that rule are rarely made (and not allowed on any of the consoles), this really isn't much of an issue. There are a variety of reasons why the laws that get passed always get struck down in court, usually relating to either:
1) no proof that the games cause actual harm to minors. Or
2) use of the ESRB rating system would give governmental autority to a non-governmental agency.
That's not to say that most retailers won't sell M-rated games to minors, though. The ESA can make live pretty rough for any retailer that gets caught selling to minors, and as such, the retailers will likely fire anyone who does get caught.