Human females have a unique and totally differentiating factor from nearly all other animal life; their bodies cease being capable of pregnancy approximately half way through their life cycle. This natural change to sterility (menopause) does not happen in the animal kingdom (nor in human males) essentially so long as they live (ok, actually there may be a couple of whales and porpoises that may also go through menopause...but I digress). Animals and male humans are still able to reproduce nearly until the end. But not human females. Even before menopause fully takes over, typically around 50 years of age, fertility rates drop radically after 40 and miscarriages surge among those able to get pregnant. By 45, pregnancies essentially cease.
What the hell does this have to do with economics, you may be asking yourself?
Judging the size and change of humankinds population is quite different than any other species on earth because of this truncated period of fertility among human females. Thus, to gauge the direction of our species, and the future consumption and potential economic activity, we must focus on annual births versus the 20 to 40 year-old female population and understand that the post childbearing, 40+ year-old female population is, from a fertility perspective, simply an inert echo chamber. The 20 to 40 and 40+ year-old populations shown below through 2040 are not estimates or projections but actual persons which already exist and (absent some pandemic, world war, or change in life spans) will slide through the next 20 years.