That's just at the ticket booth though. I'm 15 and I've seen plenty of R rated films in theaters. Buy tickets for a PG-13 movie, and walk right into the R film. The kids that work at the theater are only 2 or 3 years older then us, they don't really care.
Again, it's up to the parents. Now personally, I've told my mom that we've snuck into a couple R rated films. Was she happy? No. Did she sue the company like that crazy mother who sued Rock Star games? No. The point is there is a certain level of trust and maturity a parent see's in their child, my mother must see a high one. Some other parents may see a low one.