The MPAA rating system is entirely voluntary (well, not entirely since it may be difficult to get distribution if you aren't rated).
Nothing is mandated by the state, however; theatres are free to carry or not carry unrated films, at their discretion. The United States is the only country in the world, to my knowledge, that does not have a government mandated media ratings system. And this is something to be proud of.(well, not entirely since it may be difficult to get distribution if you aren't rated).
I don't know how it is in your town, but in mine minors have no difficulty in obtaining cigarettes, alcohol, and adult videos--not to mention marijuana, crack, heroin and the services of prostitutes. If your town is different, I envy you.Does this apply only to video games, or does it also apply to cigarettes, alcohol, and adult videos - commodities legal for adults, but illegal for children?