I am 75 and a 1K and I wear Nike jogging paints, nice T shirt, and runners. I usually fly up in front when I can, I have never been treated rudely or questioned by any UA staff on the ground or in the air. I think one's demeanor and personality play a big part.
I respectfully disagree. There were plenty of times I was boarding LAX-SFO on a saturday morning with my Cal t-shirt and jeans and a smile on my face because I was going up to watch what, at the time, was a great cal football team, and would still get stopped in the boarding lane despite my "good morning" to the gate agent as i handed him/her the boarding pass.
edit: on 2nd thought maybe they just didn't like Cal