This, extremely counter-intuitively, doesn't actually change anything at all in terms of population change. All it does is cause some brief noise as you hit the 'equilibrium point.' It's easiest to explain this with an example. Let's take it to a ridiculous extreme and say people live to 200 years old, yet otherwise maintain the same general fertility trends of having about 20 years of peak fertility. And to keep things simple, we'll assume a fertility rate of 1, which means the total population would halve every generation. We'll also assume everybody has all their children at age 20, and then dies at age 200.

So start at the bottom. And say we have exactly 2 twenty year olds. Since there are 2 twenty year olds there must be 4 forty year olds, 8 sixty year olds, and so on. This generalizes to there being a total of (2^n) people of age (20*n). So there are 512 people of age 180. The entire population is 512 + 256 + 128 + ... 1 = 1023. So what happens when this 180 year old generation dies? Your population declines by half. What happens twenty years later when the 256 population group dies? Your population decreases by half. So each generation, you start seeing an exponential halving of your population.

So how long people live doesn't actually matter. All it does is add some noise/delay as you shift from an equilibrium between fertility rates. And once that equilibrium is reached, your population will shift by a factor of fertility_rate/2 every breeding generation (~20) years. This holds true whether people live for 40 years, or 40,000. So with a fertility rate of e.g. 1 your population will exponentially decline by 50% every 20 years, until you go extinct or start having babies.

Life expectancy is basically flat for people over 80.

Lots fewer babies are dying, a good amount of young adults are staying alive, some middle age people are living longer, and a handful of older folks are too.

But once you get to 80, 90, your life expectancy is within a few years of where it would have been 100 years ago.

There’s absolutely no evidence that medical technology has made any impact on the maximum age humans can live.

And any impact it’s had on the raw numbers making it past 100 are utterly dwarfed by the kind of factors affecting global population. Like tiny, tiny percents of the population are in this “ nouveau old”.

Average life expectancy is what matters.

We'll probably have more outliers of wealthy/healthy people living longer, but poor management of broader access to basic health and education may not result in that drastic an increase in overall average results.

last thing i heard was that life expectancy was actually dropping in the developed world. Due in part to underinvestment in health services and exacerbated by COVID..

However that could have been UK specific data.

*EDIT*: It was global, data comes from the Lancet, which is considered a strong source by many: https://www.thinkglobalhealth.org/article/global-life-expect...