A fun thing about fertility rates that most don't know. They also determine the exact age ratios within a society. Imagine a population has a common fertility rate of 1. That means each successive generation is half as large as the one prior. And we can approximate the age of fertility as between 20 and 40. So now let's start with 1 newborn and we can work backwards from there.

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1 new born ->

2 20-40 year olds ->

4 40-60 year olds ->

8 60-80 year olds ->

16? 80-100 year olds

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Just ignoring the 80-100 year olds, we end up in a scenario where you have 6 people in the working age for every 8 people of retirement age. And if life expectancy inches up, then it may be closer to 6 working age people for every 16+ retirees.