The difference is teens are much more shithead-like than 20-somethings (not that I haven't noticed the increasing prevalence of 20-something shitheads), and teens are often not held responsible (legally or financially) for their actions (further enabling shithead behavior).
It's a vicious cycle where the more sheltered and unprepared for adulthood they get, the longer you want to wait to give them the freedom and responsibilities needed to grow the f*ck up. And yes, a car is often an enabler for a lot of other activities that are helpful in that respect. Just after WWII we had boys as young as 14 sign on to go to sea for months at a time with no mom or dad to look after them. They grew up, fast. Maybe not the best idea today since you'd want an education, but for example this winter we had a dog sled race here. Teens as young as 15 responsible for their own dog sled team over 400km, they got caught in a big storm and had to dig in and eventually rescued. I'd be shit scared if I was 15, alone, in the mountains, with a storm blowing and there was no one else to rely on. Of course they were prepared and not exactly our average teens, but most teens could handle a lot more responsibility than they do.
I'm not going to pretend that I'm innocent there either, not taking any responsibility is convenient. You have to be pushed a little to stand on your own, participate in chores, make your own choices and not least of which dealing with the consequences. Like using your allowance for ice cream and realizing you don't have any left for candy on Saturday. If you manage to cry your way into having candy anyway, you're not learning. It might seem kind to you then and there, but it's not doing you good in the long run. The same with parents that always drop everything whenever their kids want something, they must to respect that you have a life too and not a slave that'll come at their whim. Put them in a car and they'll still expect the world to revolve around them and is reckless about the consequences because they expect someone to clean up the mess they made.