Middle age is usually thought of as an individual characteristic and is generally defined as that period of time between youth (however you define that) and old age (however you define that). But there is an aggregate perspective, as well, as Marilyn vos Savant (reportedly the world's smartest woman--I'm not making that up!) pointed out in this week's Parade magazine (a ghost of its former self, but it still shows up in the Sunday paper). It is reasonable to define "middle age" as that age at which half of the population is younger and half older (otherwise known as the median age). By that definition, the middle is a lot younger in developing countries with high fertility than in countries with low fertility. We know this because birth rates are the principal driver of the shape of the age structure. So, in the West African country of Niger, which I noted a few years ago had the world's "worst" demographics, and where women are having a whopping 7.6 children each, the median age is 14.8 years. Uganda, Chad, and Angola are close to that.
At the other end of the extreme is Japan, where the median age is pretty close to what we think of as "middle age" at the personal level--46.5, and where women are having only 1.5 children each. Close to Japan are other low-fertility countries including German, Italy, and Portugal. The United States is several years lower at 38 years, whereas the world average is 29.6, which is almost exactly half way between the youngest and oldest median ages. These data, by the way, come from the Population Division of the United Nations, and they are consistent with what Marilyn vos Savant used in her short piece, so it is good to know that the smartest woman in the world understands where to go for demographic information.
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