The only people who are cool are the ones who don't give a toss if they are or not, and that's true of any age group. My teenage kids wouldn't be seen dead in "designer" or "branded" clothing because they think it's embarassing and shows a low level of understanding about the way in which firms like Nike and Addidas produce their gear. Some people think that's very cool of them and some people think they're prats bcause they're not draped from head to foot in dodgy sports clothing, so as zacmaster says what's cool to one person is embarassing to another.
Find your own flava, go with what suits you and don't worry what anyone thinks. Cars are subjective again, they can be cool because they are funky ( Fiat 500) or cool because they are a beautiful bit of kit (Aston etc). My one son has a lotus elise which is a gorgeous little car and the other has a mini covered in fur fabric ( yeah I know) but more often than not it's the mini people use the cool word about.Just go with the flow, you can't go wrong .