No difficult economic terms, no tough charts, just simple math.
1 - The worlds population of under 40 year olds (excluding Africa) has essentially peaked (chart below...bars represent 0-40yr/old population, dashed lines UN future estimates). What is interesting about the under 40 year old population is that they are responsible for about 97% of all pregnancies / births. It's not impossible for 40+ year old women to have children, just statistically very rare (particularly outside the developed world).
Ok, we've established the global under 40 population (excluding Africa) has essentially peaked...now we lay out the chart below that a shrinking population (above) isn't replacing themselves. Chart below shows world fertility rates, again breaking world fertility (ex-Africa) from the African fertility rate. The world (ex-Africa) has fallen below the 2.1 births per female replacement level...and even Africa is rapidly slowing.
A flat to shrinking child bearing population that is not reproducing at a rate to replace themselves and the fertility rates continue to fall. This all points to the potential the low UN 0-40yr/old population estimate could be fairly accurate (chart 1, lower bound). With either the medium or low estimate, the UN is telling us they expect a massive depopulation of under 40 year olds world-over. Somewhere between 1 billion to 2.5 billion fewer under 40yr/olds by the turn of the century & perhaps well in excess of a 50% decline (except for Africa?!?). I lay out why the Ex-Africa approach to viewing global economics makes sense, HERE.
The next three charts show annual global population growth, excluding Africa (the charts show average annual population growth per five year periods).
The chart below is focusing on the changing nature of the annual global population growth.
Finally, a quick look at select years to show the changing nature of the global population growth...shifting from nearly entirely growth among the young to declines among the young only somewhat offset by the elderly living far longer.
Of course, the flipside is the 40+ year old world (ex-Africa) population is set to continue soaring (chart below). Unfortunately, by age 65, the population consumes at about 70% of it's peak earning years...and by 75, consumption falls to somewhere around 50-60%. The elderly are credit averse, have made their major life purchases and spending (kids, homes, college) and turn to net sellers in retirement. Absent a growing population of young to buy their assets (IRA's, homes, etc.), we have a small problem (central bank asset purchases to the rescue). As for Africa, the population growth there generally consume at a rate of 5-10% the consumption of the depopulating young they are replacing. Global economic activity and consumption are likely to fall off a steep, high precipice.