The song was Joel's response to a conversation in the recording studio with Julian Lennon and a friend who had just turned 21. They were in the studio that day and stopped to say hi to Joel. The friend was lamenting how hard it was to turn 21 in 1989 and suggested to Joel that it was much more difficult growing up in the 1980s than the 1950s.
Joel, a life-long history buff, was surprised by the young man's views. His lack of understanding about the turmoil of the 1950s and 1960s, which included momentous events like the Korean conflict, the Civil Rights Movement and the Cold War, sparked Joel to record a mini-history lesson in music covering forty years, from 1949 to 1989. In less than five minutes, starting with 1949 (the year Joel was born), he mentions 117 important people, conflicts and other historical events, fads and cultural touchstones.
Haters everywhere but I don't really care.