They are in a better position to sustain that population compared to less developed countries then why are they rapidly aging?
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A lot of Less Economically Developed Countries have huge parts of their economies dependent on farming/herding etc and also have high infant mortality/low life expectancy and worse medical care. So having lots of kids means having helping hands with working the land, and having backups in case some of the kids die. Throw in religion/conservative societies and lack of birth control and you get rapidly growing populations.
More Economically Developed Countries are basically the opposite. Service based economies with low infant mortality and higher life expectancies. Couple that with less conservative societies, abundant birth control, and better medical care means sex in not primarily for procreation anymore and people live longer.
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