[King Prout]

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

[shubi]

I believe that a biologist calculated the maximum ability of the organs to persevere. I do not remember how long ago he did that or what it was based on.

[js1138]

Even today it is difficult to confirm the age of people who claim to be more than 100 years old.