Most people tend to view teenagers as potential troublemakers. It's not fair, but that's the way it is. Heck, I didn't like teenagers (as a group) even when I was one, not so long ago. Of course, regardless of how you feel about teenagers, treating them this way is not any more acceptable than treating a black person as a potential criminal just because he is black. Ageism, like racism, is a form of discrimination that's just wrong.

It may be possible to mitigate some of this effect by interacting politely with people: they'll be relieved and pleasantly surprised to talk to a teenager who is not another surly mumbler avoiding eye contact with non-peers. Sometimes speaking in complete sentences does wonders. So does a smile, coupled with a respectful but confident demeanor. There will still be jerks out there who will treat you like crap no matter how good of a person you are, but types like that do not reserve this sort of treatment for teenagers only.

As far as that particular store goes, I'd avoid it if I were you, and suggest to everyone you know that they do the same. If you really feel like it, you could go there with a bunch of friends and be loud, obnoxious and unnerving. That will certainly reinforce the stereotypical view of teenagers, but will at least annoy the nasty shopkeeper.