Terry H. answered 05/23/19
Special discount rate right now for this Duke/Emory grad!
This is a complex question, but television sets, affordable cars, rock'n'roll, home appliances, and a prosperous middle class combined with Hollywood to create the "teenager." Before WWII, adolescents were considered children who were learning to be adults. In many cases teens had to take on adult roles. But the idea of a teenager as a separate demographic with buying power and identity had not existed.
By the mid 1950s, a few interesting things had changed. Cars were increasingly reliable and affordable allowing more families to live in suburbs away from their city jobs. Farm technology had been replacing workers and the G.I. Bill provided opportunities for millions to get higher paying jobs, leaving rural life behind. Now in suburban houses, which needed less upkeep than a multi-acre farm, and with modern household appliances and packaged foods, the need for children to help out at home was greatly reduced.
Enter Hollywood movies and television. Parents with adolescents were generally more content to stay at home watching tv than to run around at night or on weekends with their kids, who were increasingly more restless as they grew older. While a kid could sit watch TV for hours, teenagers became more board with family entertainment and became the primary consumers of film, once the industry figured out that their falling sales were due to television. B movies became more popular than the actual A movie feature it accompanied, and soon Hollywood began to aim its films toward a group of kids who were able to drive the family car (or their own) and had extra money to spend. Horror, Science fiction,comedy and action films became the norm.
The impact of the content of the films is hard to judge. However, the teenage protagonist became very commonplace, giving the teenager a sense of group identity and, in many cases, superiority. For the first time adolescents as a group were depicted in movies (not all) as knowing more than adults and needing their own music - rock'n'roll- and their own cultural status. "Cool" became a concept, and "squares" who always followed their parents and society's rules proved uncool and could be justifiably cast out.
Magazines, radio, and multiple industries took heed and began marketing new products to teens or old products in new ways. What made film so powerful was that not everyone in remote areas had television yet but could usually get to the movies on occasion. Images of James Dean as a rebel without a cause and Brando mumbling his way somberly through dialogue exposed many movie-goers to odd, new role models.
As the adolescent film-goers became adults, Hollywood followed the money by making grittier films that appealed more to the 1960s adults, introducing into mainstream films foul language, nudity, homosexuality, graphic violence and topics once considered perverse. This goes beyond the question, but it's important to realize that, while appealing to a new group of teens in the sixties, they continued to focus on this baby boomer group who they helped to create.