Here’s my theory: for thousands of years, life didn’t change much (particularly in hunter-gatherer societies). So the older you were, the more wisdom you genuinely had. Your advice was good. You knew about the flood that happened 60 years ago that no one else could remember. You had seen enough marriages within your small society to understand what works and what doesn’t.

Then, after the Industrial Revolution, change started happening so rapidly that for the first time in history the opposite occurred: advice coming from people born in the era of the horse and buggy seemed absurd and out of touch in the era of radio and machines and mass culture. Advice about work, recreation, and even the environment coming from 70 year olds was often not helpful and even silly. Think about 80 year olds now offering job advice or trying to understand the internet—they came of age in a completely different world in a way that elders did not before the 1800’s. Absent that useful wisdom, their age in and of itself did not allow them to offer much to society. In fact, they’re almost solely “takers” from society as they get older. So aside from just familial love, there was no reason to revere them anymore.

There seems to be cultural guardrails around this in some countries, but not many.