This promises to be an interesting discussion! I think that the main thing that teens don't understand about their clothing choices is that outfits need to be appropriate for the place and activity where they are worn. For instance, short shorts and crop tops may be fine at any age if you are spending the day at the beach, but would never be acceptable for most workplaces. It seems to me that many young people try to push the envelope by wearing clothing in the wrong venue; I see students wear their dance club clothes to school, go out shopping in their bathing suits, and so on.
As far as having certain clothes work for a certain physical appearance, that is more a matter of buying the correct size than anything else. Girls seem to prefer clothing that is two sizes to small, while guys tend to buy things two sizes too large. Matching the clothing size to your actual physique is usually the best bet.