Until the early 20th century adolescence as a category didn't exist. Children dressed like adults, talked like adults, and wanted to be adults. There was no in-between. Interest in adolescence and youth culture really took off in the 1960s, a time when teens and young adults developed their own style. Their clothes, music, slang, and even ideals were drastically different than their parents', and they were identified as their own group.
Now the Internet has changed everything yet again. It's given us access to any subculture we want to join, so we can be in the know long after college comes to a close. Ask one 28-year-old about her favorite music and it may sound like she stopped tuning in five years ago; ask another and she'll tell you what came out last week.
Of course, age is defined differently for everyone. Do you act younger or older than your age?