The Song was Written After Joel Talked to a Friend of John Lennon’s Son
Joel was 40 when he got the idea to write and record the song. He says that he wrote the song after having a vehement conversation with a friend of Sean Lennon, the son of John Lennon.
The friend confessed to Joel how he thought it was a terrible time to be 21, to which Joel responded in a reaffirming tone that he could feel what he is going through, as he too went through a lot himself when he was 21.
Source: reddit.com
The friend didn’t think Joel could relate to his pain and invalidated his struggles during the 1950s. That is when Joel found it pivotal to write the song and make the new generation cognizant of what happened before they were born.