In my experience (still a teen...) it's the rule until proven otherwise. I mean just by being a teen many people assume you have some level of technical knowledge, but it doesn't go beyond helping people with setting up their computers and phones...anything like programming is like an "oh, really?" to a "oh, wow [you can actually do it]!" thing.

Well, to be specific, that was the case for me from 13 to 16, then I started prefacing that with "I'm majoring in computer science" and then the discrimination started to go away.