Why do adults underestimate kids /teens?
I don't think they do. I think most adults overestimate kids, particularly their own.
Just look at all the dumb questions from kids/teens on YAnswers and in this category. Yes - kids know about lots of things that olders don't (mostly tech, drugs, today's "music"/clothes/lingo and such). But they display virtually no common sense or the ability to problem-solve simple solutions. . . . same as when I was a kid and my grandparents couldn't tune the tv and didn't know anything about pop culture of the time. What scares me the most is that so few know anything about history, civics, government, budgeting etc.
That's a foolish assumption. My 13 year old granddaughter is is bordering on genius.