The gist of the Crazy Years has been with us for a long time. Remember, Heinlein grew up when cars, telephony, radio, electrification, and more were transforming culture round him. The phrase itself almost certainly refers to Années folles -
https://en.wikipedia.org/wiki/Ann%C3%A9es_folles .
To phrase it a different way, when weren't the Crazy Years? I first heard the comment that we are living in Heinlein's Crazy Years back in the 1990s.
Or, from https://en.wikipedia.org/wiki/We_Didn't_Start_the_Fire#Histo... :
> Joel got the idea for the song when he had just turned 40. He was in a recording studio and met a 21-year-old friend of Sean Lennon who said "It's a terrible time to be 21!" Joel replied to him, "Yeah, I remember when I was 21 – I thought it was an awful time and we had Vietnam, and y'know, drug problems, and civil rights problems and everything seemed to be awful." The friend replied, "Yeah, yeah, yeah, but it's different for you. You were a kid in the fifties and everybody knows that nothing happened in the fifties". Joel retorted, "Wait a minute, didn't you hear of the Korean War or the Suez Canal Crisis?"