Everyone who has gone through puberty knows the monumental changes that accompany it. It seems like you are entering an entirely different body, and an entirely different stage of life. But before the 1900s, there was no time period considered "adolescence"; you simply were a child until you were an adult. Even if you were seen as a young adult, you still had to work, get married, and in a lot of ways undertake the burden of adult responsibilities. While that is still true around the world today, in America, your teenage years are no longer seen as adulthood and teens are not legally or socially recognized as "adults."
Teenagers is a term that is so frequently used today that it's hard to believe people could have lived without it. Yet, the concept of adolescence is a lot younger than most people would believe. In truth you could say we owe it all to the first man to study teenagers, G. Stanley Hall. His book, Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion, and Education was published in 1904 after Hall studied a wide range of adolescent theory. He took the time to try and understand this transitional period and all of the emotional, psychological, and physical changes associated with it.
The spark set by G. Stanley Hall prompted an academic fascination with teenagers which then became a popular topic of research among psychologists. More and more people were studying teens and publishing their findings. The term "teengager" was not yet coined until the explosion of youth’s culture and their "rebellious nature" in the 1950s.
The evolution of youth culture is easy to trace through the 20th century. If you want to know what the kids of the day are doing, look towards what the angry adults published in the news. These criticisms by adults who misunderstand or dislike the younger generations is no new thing. Cartoons in the magazines are also great references to see each generation’s personal culture and how it differed from before.
Teenage subcultures are also reflected in the marketing directed towards them. Adolescents have been directly targeted in marketing since the start of the 20th century. This doesn’t even have to mean through sales, but with the invention of cars and the lowering of social constraints, people could go to events and places targeting young audiences. Dance halls, movie theaters, even malls later on all became safe havens for blossoming youth cultures. This is around the time that the term “teenager” started to emerge: When there was a clear divide between the ages and younger adolescents that had their own personal world separate from adults.
When the youth of the world gained mobility and could move about without any older supervision, that’s when this separation of cultures really solidified. It's hard to transition to adulthood no matter if you’re considered an adult or a child in the process. Adolescents formed groups within themselves to find comfort by comiseration. Not being required to work or get married, having access to cars, and lower social restrictions on activities between the two sexes created what we now consider a proper "teenager." Youth culture will always be criticized by the generations that come before, but teenagers are unique emotionally and physically and aren’t going anywhere.