> retirement has existed really since humans lived past their 40s

We've always lived past our 40s. We just stopped having a bunch of people die in childhood (or birthing children).

https://www.bbc.com/future/article/20181002-how-long-did-anc...

"If one’s thirties were a decrepit old age, ancient writers and politicians don’t seem to have got the message. In the early 7th Century BC, the Greek poet Hesiod wrote that a man should marry “when you are not much less than 30, and not much more”. Meanwhile, ancient Rome’s ‘cursus honorum’ – the sequence of political offices that an ambitious young man would undertake – didn’t even allow a young man to stand for his first office, that of quaestor, until the age of 30 (under Emperor Augustus, this was later lowered to 25; Augustus himself died at 75). To be consul, you had to be 43 – eight years older than the US’s minimum age limit of 35 to hold a presidency."

"As a result, much of what we think we know about ancient Rome’s statistical life expectancy comes from life expectancies in comparable societies. Those tell us that as many as one-third of infants died before the age of one, and half of children before age 10. After that age your chances got significantly better. If you made it to 60, you’d probably live to be 70."

Neanderthal elders probably took on lighter-duty tasks as they aged just like today's elders do.