Ooooooooh! This is really, really interesting! And helpful. Especially the suggestion to abandon the idea of generations. Though age cohorts probably still make sense. (But then again... it probably does matter if your parents remember events from four thousand years ago, versus if your great-grandparents can only tell you about the last century.)

hint: not exactly like modern American nuclear families,

Why's that?

also no undying elves complaining about how their race is dying out because they only have a kid a century each

...At the very least, not without an ongoing pandemic or Thor trying to end you. Or they could be very, very warlike...

(Or fertility could be declining and they're extrapolating trends that won't even begin to affect them for another few thousands of years...)