That model might change with how social media and propaganda seems to be messing GenZ boys up. Like I used to think boomers would die and then we could take care of climate change as the new generation rises, but that hope has gone away because republican propagandists have gotten gen z boys.
That sounds right - a further complication. But life expectancy of 47 in 1900 coupled with half a dozen kids per family contrasted with 82 life expectancy and a couple of kids per family makes for an incredible slowing of social change.
Thing is that in 1900 about half those kids wouldn't live to reach adulthood, and all those juvenile and infant deaths were what dragged the life expectancy down.