I had this happen to me a few times, but it has slowly gone away (or I just don't care to notice anymore).
I'm 21, and I bought my first gun at 19. I would walk into gun stores and everyone would look down their noses at me and it honestly really bothered me for a while.
I felt that I'm A. of legal age for purchase, B. responsible and willing to learn, and C. my money is printed on the same paper as anyone else's.
My ex girlfriend's stepdad and I used to go shooting a lot so we would go to gun shops on occasion and he knew most of the guys, so being seen with him I guess may have changed their outlook on things.
Not sure, I'd pay it no heed, be firm, but don't be a jerk.
Tell them how you feel, don't show fear. You have to realize though, do you know how many jokers come through their doors and act like fools? Too many, way too many. They're wary and they've become prejudice. Prove them wrong. Don't be a smartass, be humble, and treat them with respect.
If they refuse to sell something to you that is legal to be sold, tell them "I'm in your shop because I chose yours out of anyone else's. I'm here to spend money, to support your business and brick and mortar gun stores everywhere. I'd really like to make this purchase here, but if you don't want to sell to me, that is fine". If they still decline, go somewhere else, someone wants your money, I promise.