No, here in the US the regulation of game sales is something handled by the stores. What that means is that at the register the store can ask you to provide ID to show that you're old enough to buy the game, and if you're not they can choose to deny the sale. But they're not legally obligated to do this. Most follow these rules because consumers tend to appreciate their compliance, so it becomes corporate policy.
However, parents or other people who are old enough to buy a game can buy it and give it to someone who's underage. I'm not sure if this is what the California law did, but I know that when they made one in my home state of Oklahoma, the law made it a felony to give M rated games to minors. This meant that it became not just the store's responsibility to keep the games out of the hands of minors, but instead it became the responsibility of every citizen to keep them out of the hands of minors. And unlike before, where violation of the rule meant nothing more than the possibility of some social criticism, it would become a punishable crime to break the law. Essentially, it would have moved games away from having been classified similarly to most entertainment media, and made them more akin to pornography, an "entertainment product" which can not legally be consumed by minors.
