I think the decline of younger Americans driving is because of the overall economic climate in the US, not because it's uncool. Talk to any 18-25 year old and ask if they would like to have a car if they could financially swing it, and the majority of the time they would say yes.

As a young adult myself I'd love to own a car, but it's almost impossible to have enough capital to pay for insurance, maintenance, petrol, the auto, and my student loans. I'd be digging myself into a financial ditch I wouldn't be able to get out for over 20 years.