I thought this as well, but I did a little research before responding, and it looks like even though this is broadly true, people still weren't living particula

I thought this as well, but I did a little research before responding, and it looks like even though this is broadly true, people still weren't living particularly long before the modern era. For example, in Ancient Greece, a man who lived to 15 would expect to live to 37-41 years, in Rome if a man made it to 20 they could expect to live to 60, in the late medieval if you made it to 25 you could expect to live to ~48 [0]. You still need to make it to 60 to be a great grandparent, assuming you and your kids are having kids at ~15 years of age (edit: and that might be a friendly assumption given how high infant mortality was). [0] https://en.wikipedia.org/wiki/Life_expectancy https://en.wikipedia.org/wiki/Life_expectancy