I don't get it. I really don't.
Then again, i've been flying fairly often since i was 15, had my first elite card at 17, and now i'm 21 and see more airplanes than Bradley's tower in a year. (Well, maybe not THAT many.)
Anyhow, i'm in my early 20s now, i still look like a teenager, and i've really not had much problem. The worst thing i've ever gotten was some trouble from a CO gate agent in Newark who didn't believe that was my elite card, and a quick flash of the ID (which i'm never asked for, ha ha ha! *grin*) fixed that.
I try to look decent when i'm gonna be in the air; i might be all for the sweats and a matching well-stained t-shirt in class or even at work, but for travel i try to do better than a hoodie and jeans.
I don't know what it is. Admittedly, i have my guesses, and i'm not gonna share them here. I will admit that I do occasionally get a pissy-looking exec next to me, but don't we all? I've also had some very interesting conversations with neurosurgeons, authors, and dot-com execs. (The latter tend to never be that well-dressed.

)
I think the worst experience ever, though, was a flight on HP (today is "i hate HP day") back in the day when i was repeatedly referred to as someone's daughter, and he didn't know who the bujeebus i was.
Just be nice, act dignified in the face of poor treatment, and keep track of everything in your head. When you get bad service, complain via the proper channels in a polite, dignified manner, and you'll do much better.