The decade of the 1950s saw the rise of actual "Teenagers."
While there have always been young people in their teen years, they were never really teenagers. Let me explain. In the past, few people actually graduated High School. The majority of young people were expected to go to work during their teen years, and accept the responsibilities of a full time job. Families were extended, and everyone was expected to pull his or her share of the load for supporting the family. This is why the majority of High School graduates before the 1950s were girls and not boys.
By 1950, only slightly more than half of the US population had a High School diploma (See graph). After High School only the well to do went to college. The vast majority of went to work. This meant that there was very little leisure time or extra money. But this changed with the 1950s.
Before the 1950s were the war years, and before that the Great Depression. And before that were the hard labor years when everyone worked because wages were low. But by the 1950s, well paying jobs were created in the Boom Years of the Eisenhower administration. Suddenly, the need for younger family members to go out and get jobs to support the family was no longer needed, and more young people were encouraged to stay in school. This meant that "Childhood Dependence" was dramatically increased, as young men and women 15, 16, 17, 18, etc, were no longer a part of the job market, but had more free time on their hands.
With this free time, more and more activities surrounded being teenager. As amounts of disposable incomes increased in this decade, more and more companies began marketing to this new ghroup of consumers. Clothing, Music, Fads, etc were specifically aimed at this new marketing niche, and the "Youth Culture" was born.
Much of this new culture was influenced by Hollywood which attempted to cash in on the yourth culture, by creating films aimed specifically at young people who were fast becoming the biggest attendees at movies and Drive-Ins. Films like "Rebel Without A Cause," "Blackboard Jungle" and even shlock films like "I Was A Teenage Werewolf" all attempted to cash in on this new market. Indirectly, films like these also helped to define the culture which was quickly being associated with blue jeans, motorcycles, rock & roll, and rebelliousness. It worked.
Hope this helps.