I think the age restriction helps but doesn't solve the problem entirely. As others have mentioned, jerks come in all age ranges though 99% are from the 30-below crowd (relax, I'm in that group!).
I don't have a problem with someone's phone going off or even if they take the call, just say "wait a minute, I'm at the movies" and have the person hold while you take the 30 seconds to talk outside of the theatre room.
I will say that the majority of the people that annoy me at movies are those under 21. I'd even go as far as to say those under the driving age. It is usually a group that giggles at everything, talks during the movie or consistently gets up, leaves, gets up, leaves, etc.
The biggest age group that annoys me? The babies. Parents, just because you can't find a babysitter doesn't mean that the rest of us have to put up with your crying baby. You say you don't get to go out anymore and enjoy movies? Not my problem.
Okay. /rant 🙂