(FORTUNE Magazine) – The mix of citizens in developed countries is tilting toward the old. For youth, look to the Third World, particularly sub-Saharan Africa, where families average six-plus children each (see chart). About two of ten children in the region die before reaching age 5. It's the old story: As nations industrialize and become more prosperous, their birthrate falls. Populations in many developed countries are not even reproducing themselves. The private Population Reference Bureau in Washington, D.C., says that Western Europe and Japan check in at or below replacement level -- two babies per mother. The U.S. is holding its own, due to a slightly higher birthrate, immigration, and the boomers' baby boomlet -- that is, more couples having babies. Neither birthrate extreme is good. Less developed countries must expand their infrastructures to support growing populations but don't have the economic growth to pay for it. Industrialized countries face a deficit of the workers they will need to support older generations. Despite their opposition to immigration now, Japan and Europe ultimately may have to import more workers to make up the shortage.

CHART: NOT AVAILABLE CREDIT: FORTUNE TABLE/SOURCE: UNITED NATIONS POPULATION DIVISION CAPTION: WHOSE POPULATION IS AGING FASTEST