First of all, please don't get upset at the theater employees for doing their jobs. Yes, it can be annoying to get carded multiple times, but if it's a different person, how is he or she supposed to know somebody else already checked you? Even if it's the same person, you have to remember that these guys and gals see hundreds of people in a short period of time, and the likelihood of remembering a specific person is low. Yes, I know that you had to show ID to buy your tickets and all, but they just want to make sure younger people aren't getting older people to buy their tickets for them. (I know I've been approached a few times in theater lobbies by underage kids offering me a few bucks to buy their tickets). Management probably has established "checkpoints", and the staff members are just doing what they're supposed to be doing. Maybe they had parents complain about their kids getting into R-rated movies, so they're cracking down. Just keep your ID out with your ticket until you're seated.
As far as looking younger - it was kind of weird for me. When I was in high school, people thought I was a college student. However, once I hit my late 20s, people started thinking I was younger. It was as though I stopped aging for awhile. Now that I'm 40 (almost 41), people still think I'm younger. They usually guess early 30s. I like it of course, but mostly because it's fun seeing that shocked look on people's faces when they find out how old I really am.