As far as I know, compliance with the age requirements of of the MPAA ratings system is *completely* voluntary. The ratings system is a service provided by the MPAA and has absolutely no legal structure to back it up. Theaters could theoretically be punished for exposing children under 18 to some forms of sexual content (under the same regulations that prevent kids from buying porn) but, as we all know, there are a lot of non-sex-related things that can also earn a movie and R rating that are totally legal to show to children.

That said, the MPAA does usually require theaters to comply if they want to continue receiving MPAA movies, but it seems unlikely that the police would get involved in this unless someone refused to leave the theater at the theater owner's request and the police were called to remove the person.