There was this one theater near my house when it was still Mann Cinemas that was really anal about the 17 year old age thing, which was always kind of funny when it was some douchebag from your high school. We all just started to go to the Edwards that was at the mall, which was way bigger and way nicer. They didn't give a crap at all as long as you weren't ridiculously young or anything, just teenagers and above.
Both places, however, were awful about bringing in outside food, which is dumb. To counter this at first, I tell them that I'm bringing in something that theater doesn't have at their concession stand, so they should either provide it or let me bring it in, which usually worked. If not, or if I was bringing in something they stocked, I just didn't let them check my bag or whatever, simply telling them "no." Movie theaters- or anywhere else, for that matter- don't have the right to search you anymore than the cops do without a warrant, especially when you've already bought something form them. It's like the same thing when Best Buy or whatever want to check your bags. You have the legal right to go tell them to fuck themselves and the constitution's on your side. It's hardly really that warranted to escalate it that far, but you could. Okay fine, I've done it. And it's worked.