“The young always have the same problem - how to rebel and conform at the same time. They have now solved this by defying their parents and copying one another.”― Quentin Crisp, writer and raconteur
The powerful passion of the teenager is at the heart of the dramatic conflicts in Grease. Although use of the words teen, teener, teen-age, and even teenager first appeared as early as 1899, they didn't appear in common usage until the 1920s. After 1900 reformers, educators, and legislators began to separate teens from adults and children. State and federal governments legislated minimum age requirements for sexual consent, marriage, school attendance, and work, and later for voting, driving, and drinking alcohol.
The single greatest factor that led to the emergence of the independent teenager was the automobile. Teens enjoyed a freedom from parental supervision unknown to previous generations. In earlier times, young boys and girls spent their first dates at home. The courtship process rapidly evolved into dating, which, when removed from the watchful eyes of anxious parents became an outlet for experimentation with sexual behaviors before marriage.
Automobile technology led directly to the other major factor that fostered a teenage culture: the consolidated high school. Buses could now transport students farther from their homes, leading to the decline of the one-room schoolhouse. Between 1910 and 1930, enrollment in secondary schools increased almost 400%. Graduation rates rose from 29% in 1930 to 50% in 1940. Before long, schools developed their own cultural patterns, completely unlike the childhood or adult experience. School athletics and extracurricular activities only enhanced this nascent culture. The American teenager was born.
From The Rise and Fall of the American Teenager by Thomas Hine on U.S. History.org.