Teenagers have been around for a long, long time, although not many people realized they existed as a separate age group at first. But, around the twentieth century, people started to notice that there was a weird time in between childhood and being an adult, and that the transformation between the two wasn't instant. And, when they did, it started to become romanticized by the media, and with that, people became inspired and the concept of a "teenager" was born.
Right away, stereotypes were created. There was all sorts of different roles for teens to follow, some that were admired by society and some that weren't. But teenagers often took pride in who they were, and eventually made an entire culture of their own by listening to their own music, wearing their own clothes, doing their own things, thinking their own ideas, and being their own person. A sense of individuality can be associated with the teenage years, despite the fact that stereotypes are common. America was founded on individualism, like that, which is why teenagers are so influential on American culture.