I am confident that you are not the subject of age discrimination. If I were a GA, I would probably not be impressed by your casual attire. I personally find a T-shirt, shorts and a baseball cap to be more casual than anything I would wear. Actually, I hate seeing men in baseball caps inside buildings, but that is probably my age prejudice. It has nothing to do with the age of the person wearing the cap.

I would relax and enjoy the fact that you are younger than you look. If you want to be treated with the respect you feel your academic degree entitles you to be treated a slight up-grade in attire would probably do the trick. Clearly this is an issue for you as you made sure to tell us that your title was on your credit card.

As a society we are used to seeing professionals wearing something more than shorts and a baseball cap.