Teenagers prior to World War II were expected to take life seriously. Young males were expected to join the military and/or go out and get a job in order to help bring in money for their family. Young females were taught how to take care of the household and prepare themselves to be a dutiful wife and take care of children. Also, teenagers in the late 1930s had very little economic freedom, independence, and input into decision.
In the 1950s this changed. The United States emerged from the War the most powerful and affluent nation in the world. The economy picked up and teenagers began experiencing a great deal of economic freedom and independence. The "American Dream" was born, but at the same time there was a fear of losing America's prosperity and security. The focus for much of the fear over what America was becoming was youth - it was an adult obsession and shared assumption that young people lacked the discipline and get-up-and-go that had made America great.
As the 40's ended and the 50's emerged, marked differences between teenagers and parents began to emerge. From a transformation of the dating system (going steady and early marriage become the norm as opposed to the "rating and dating" trend that was fashionable before the war), to the new medium of television gaining widespread popularity and often portraying teenagers as juvenile delinquents. 'JD's' followed the standard black leather and denim jeans look set by Marlon Brando in 1953 film, The Wild One. The widespread adoption of rock and roll also helped foster differences between parents and teenagers. Rock was loud, rhythmic, and full of energy. Adults didn't get it and even FBI Director J. Edgar Hoover called the new music "a corrupting influence". Holden Caulfield, the hero of J. D. Salinger's 1951 novel The Catcher in the Rye, was a literary embodiment of teenage angst and alienation further fueling adults perception of teens as rebels.
When heavy metal music gained mainstream popularity in the 1980s, music became a touchpoint for the Generation gap between the parents, who were often teenagers in the 50's, and the younger generation. The 1980s saw Tipper Gore, wife of Democratic Presidential nominee Al Gore, launch a crusade against raunchy and violent rock lyrics, and heavy metal was one of her main targets.
A seeming generation gap may be present between different generations, as well. Starting with the fear of childbirth, people may learn or otherwise impart a fear of children, fear of youth, and/or fear of elderly people. Whether favoring the perspective of adults or actually solely allowing the perspective of adults, society may seem to also foster gerontocracy, which pits elderly people against children, youth and adults, as well.
The largest generation gap, though, separated those born after 1940 from those born before 1935. It was summarized in 1967 with the slogan among the young: "Don't trust anyone over 30." As this gap moved through the decades, it separated those who appreciated rock music from those who did not, and the computer literate from the illiterate. Those on the old side of the gap could not wait to enlist in the military and fight World War II or the Korean War, while the young challenged the military draft during the Vietnam war. Despite the radical changes in the electronic and technological environment in the last several decades, a defined gap does not separate today's generations as it did in the sixties and seventies. The left/right political delineations in the U.S. at the present tend to mimic the generation gap of the sixties, but not on the basis of age.