even when “the elderly” meant folk in their 30s and 40s…

I‘ll stop you right there. At no point in human history was someone in their 30s or 40ss considered elderly. Statistics that claim an average life expectancy of 40 years are heavily skewed by child mortality. If you made it past 5 years old, your top causes of early death were war for men and childbirth for women. Survive/dodge that and there was a decent chance to make it past 70.

That applies about as far back as we can estimate individual people‘s lifespans. For example Ramesses II ruled for about 65-70 years and it’s assumed he became Pharaoh in his 20s, making him about 90 years old in total. Plato made it to at least 75. There are many more examples throughout all of history.

Did we ever get an explanation for that?