Two thoughts immediately come to mind on that:
1. It wasn't really until after World War I that the United States began to dominate the world stage and therefore play a major role in setting the tone.
2. Mass communication and easy movement across regions is probably an important element in creating a widespread national "meme" for an age. And the late teens are when those really started to come together with radio and movies making it possible for the entire country to be simultaneously consuming the same popular culture and the cheap automobile allowing for a level of personal freedom in movement hardly dreamed at before.
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