You asked the theater about that? I'm sure they would have different opinion.
And your ticket has a movie on there, correct? I'm sure it's assumed in the eyes of the law that your ticket is a contract exectuted whereby you get admission to said movie printed on the ticket in exchange for the money you gave them.
But, then again, maybe it's not illegal. I mean, theaters are not legally responsible for keeping kids out of R rated movies. If the theater doesn't care and allows kids in these movies, they aren't breaking any laws. So, maybe you wouldn't be breaking any laws here.
Anyway, I don't have a problem with it personally...I've done it before, when I was younger.