I think it's downstream of the rate of change in society driven by technology.

20 years age difference has never been so meaningful to economics and culture.

There's some precedent with major geopolitical events, the greatest generation via WW2 etc, and the boomers they created... but even the scale of that was technology driven.

There's still people alive today that were once in awe by horseless carriages, it's entirely rational to demarcate the cohorts.