As most people have already said, this sounds like yet another case of bad service and not age discrimination (though that may be indeed be a factor).
I am even younger (just turned 22) and look quite young as well. I've been in a few sticky situations and I can safely say that, while it may not help sometimes, being nice and staying calm never hurts. Dressing well probably helps, but being cool gets you so much further.
UA has generally been pretty decent to me in airports worldwide, maybe because I travel quite a bit with my family (dad is 1KMM, mom is 1P).
Take it easy on the OP, people. I empathize, as most of us have been in a similar situation at some point. Besides, MBAs get enough flack as it is. Reminds me of this
FedEx ad!