Teens are a pain in the butt at the movies. I'll be the first to admit this. As a theater manager, one of my non-described job requirements was babysitting. But this is a task that sounds easier than it is. For many kids that we would kick out of an auditorium, or have removed from the lobby or parking lot, we would have to deal with angry parents who don't like other people disciplining their sons and daughters. No father wants to hear that his 15-year-old daughter was involved in sexual activity in a movie theater, but if he holds it against the people who discovered her, or against the theater in general, then it's no longer worth the trouble and the complaints. At that point it becomes easier to just let the kids have a run of the place.
Unless you have a laid-out, documented policy against certain activities in your cinema, there exists a problem of how to handle situations. This is probably the reason that some theater chains are in fact making new rules and regulations in order to properly deal with teenagers, who tend to go to the movies just to have a place to hang out, whether they're inside or out. One chain in particular, Kerasotes Theatres, made headlines recently for its controversial ban on teens, who are no longer permitted at late-night shows on Fridays and Saturdays (they call it "Adult Friendly Shows"). Unless they are with a parent and have a special Kerasotes-issued ID card, kids 16 and younger can't attend movies showing after 9pm.