The developed world is getting old — maybe too old. As this map from the Pew Research Center shows, the percentage of the population over 65 is getting really high in Japan and much of Europe. "At least one-in-five people in Japan, Germany and Italy are already 65 or older," Pew's Gretchen Livingston writes, "and most other European countries are close behind."
The problem with aging populations is pretty simple: they're hard to pay for. When people age out of the workforce, younger people are supposed to step up and contribute taxes to the programs that support their elders. But when you aren't having enough kids — which is really the root of the aging problem, particularly in Japan — the whole system gets really creaky.
And very young countries — such as nearly all of those in the Middle East — have their own set of problems. It's tough to generate enough jobs to help all of the people entering the workforce: according to a UN report, 80 percent of unemployed Egyptians in 2006 were under the age of 30.
Having huge numbers of unemployed young people isn't exactly a recipe for stability. Arguably, the Middle East's "youth bulge" was one of the key factors behind the 2011 Arab Spring uprisings that overthrew four governments and transformed the region's politics, but also led to violent chaos in countries like Syria and Libya.
In other words, national age is a Goldilocks problem. Country populations shouldn't be too old, and they shouldn't be too young. To avoid demographic problems, a country's age needs to be just right.