The ratio of younger vs older people is also a function of biological aging which might look very different in 500 years. I don’t think we can reasonably expect to retire at 65 if healthy lifespan hits 200+.

If 150 year olds are as healthy as current 50 year olds they may very well be expected to work. And personally I’d happily extend how long people are expected to work in exchange for significantly longer lifespans.