That has never changed, even though medicine has raised the mean and median age limits. Once you survive infancy, plague, war, pestilence and bad genes, technology doesn't contribute much.
My father is 96. He survived a hip fracture two years ago and is still healthy. He hasn't required any high-tech medical help, other than the pin in his hip.
I once heard that the "firmament" was a layer of water above the earth that contributed to humans, animals, and plants living longer and being larger.
it's a puzzlement.
there are three basic options
1. wild-assed guess, that turned out to be accurate
2. there were enough people to make it to about that age to be observable, and a general paradigm was deduced
3. "somehow, they knew"
I don't buy #1
I lack data by which to decide between #s 2 and 3