The ranks of people from 20 to 80 are increasing rapidly, something that has never happened in history. Once that stops happening, and the world population profile resembles that of the U.S., Europe and Japan, overall growth must stop. There will be no more ranks to fill out.
It sounds stupid to say this, but you must remember that people are only added to the population at age 0. After that, they gradually disappear. We don't think of it that way in the U.S. because much of our population is from immigration. People are added to our population at any age. In a first-world country where the birthrate has been at replacement level for decades, the population profile is very different from a third-world country where it only dropped to replacement levels 20 or 30 years ago, which is when most of them did. There will still be a great bulge of people under 30. Population will increase until the people who are now 0 to 30 die, in the future. Of course, the 0 - 20 cohort might start increasing again, so that projection may be wrong. But it seems unlikely the birthrate will increase. Quality healthcare and contraceptives are widely available, and healthcare and longevity are increasing in every country except the U.S. (Our healthcare and longevity are falling because of the opioid crisis, for political reasons, and because the healthcare industry is gauging the nation. These trends could easily be reversed.)
World population and the population for different countries is shown here:
https://www.worldometers.info/world-population/
Life expectancy is here; you can see it cannot go much higher before it reaches the natural limits. That will end population growth, unless the 0 - 20 cohort begins growing again:
https://www.worldometers.info/demographics/life-expectancy/
The lowest is 54 years, which is more than you probably expected. People have the notion that third-world life expectancy is stuck at 20 years, or something like that. That was only true in premodern conditions when infant mortality was very high. Even in Medieval Europe, a person who survived to age 6 was almost as likely to live to 60 as we are today. There has also led to the myth that people looked old, or suffered from the diseases of old age, at around age 40 in ancient times, because that was the average lifespan. (https://www.verywellhealth.com…hroughout-history-2224054) That is nonsense. A person in good health at age 50 or 60 in the year 1526 was a vigorous and healthy as a people are today. You can tell by looking. This guy was 57 in 1526:
https://en.wikipedia.org/wiki/…brecht_D%C3%BCrer_078.jpg
He looks pretty good to me. As my grandmother would say, his color is good. In those days, adults tended to be in good health, or dead. There were plenty of 80-year-olds back then. People did not drop dead at age 40!
Modern population profiles still vary a great deal. The population profile in Russia still reflects the losses from WWII.
