The youth culture that arose in the United States after WWII had its origins in the prosperity that the country was experiencing at that time.
After WWII, American youth had, for the first time, a great deal of money and a great deal of leisure. In past times, youths went fairly directly in to the workforce because their families needed them to. They did not have much in the way of money to use on consumer goods and services. After WWII, prosperity came to the US and youths had a great deal of money because their parents could afford to give it to them. They also had a great deal of leisure time because they were remaining in school for much longer.
Since youths had money and time, they were able to develop a youth culture in the postwar years.