Life expectancy is heavily skewed by infant mortality, which was of course rampant.

If you made it past your tenth birthday, you had a 50% chance of making it to 50, and a 25% chance of making it to 65.

En masse, that makes for an amply large population of people in the age range for dementia to draw conclusions.

https://en.m.wikipedia.org/wiki/Demography_of_the_Roman_Empi...

Average life expectancy is a very bad number to compare across such vast amounts of time, especially when you're looking at a disease that only happens in advanced age.

Infant mortality rates were dramatically higher in the past and drag that average way down, for instance, say I have a 1 day old infant, and a 100 year old person and they both die, they have a combined 'average life expectancy' of 50 years old. But, once you made it out of the 'childhood illness' mortality stage, most folks could expect to see 70 years old.

https://sc.edu/uofsc/posts/2022/08/conversation-old-age-is-n...