This is something that used to always get under my skin as well. I started business travel at 23, and was always the youngest in priority and first class. I am usually one of the younger ones now at 28, however it's gotten better.

I used to find that almost every single time, when I would walk up, the gate agent would make an announcement and say, We are still/just boarding first class. Not necessarily directing it at me, however I am the reason it was triggered in their head. (You forget to lock your car, and someone walks past you and then you hit the lock button. You don't think they're going to rob you, but they remind you to double check...)

It still happens, although not as much, and now as an ExecPlat/Chairman, I sure won't let it bother me. The GA don't even bother me as much as the lowly silver members on US who give you looks; when they're practically begging and have their fingers crossed for a T-24 while mine came a week out.

Don't sweat it.