Countries with fewer young people have a higher GDP per capita, and countries with more younger people have a lower GDP. Countries with a lower GDP suffer from a combination of being poor due to too many children, having more children because they are poor and being poorer and having more children because of a third factor.
New Geography staffer Sami Karam explains the effects of poverty on fertility rates and vice versa. He then goes on to explain how some countries go on to experience a demographic dividend and the effect that this has on an economy:
Meanwhile, this is the world that we face in the next few decades: rich countries that are getting older and poor countries that are very young. Working-age populations are shrinking in the rich countries while in poor countries, they are booming beyond those countries’ abilities to educate them, shelter them and employ them.