Why are kids often so perfect in the movies?

Unfortunately the film industry always tends to have trouble characterizing teenagers and in fact many adults realistically. Have you seen Boyhood though? Aside from one scene, it uses all of it's dialogue in the most realistic sense. Or at least, the most realistic I've seen in any film so far.

"All I'm asking for is total perfection." - Lord Business

Because it's fiction written by adults. You've got to open your mind when you watch movies, any movie, because they're works of art, that's never fully realistic. And yes, there are teenagers who could speak or behave this way. It's not because you haven't yet discovered them that they don't exist in reality. Also, what makes you think these characters are perfect? It's not too hard to recognize that they've got plenty of flaws. Open your eyes.