http://www.wilsoncenter.org/index.cfm?fuseaction=wq.essay&essay_id=519403

引用：
【Something dramatic has happened to the world’s birthrates. Defying predictions of demographic decline, northern Europeans have started having more babies. Britain and France are now projecting steady population growth through the middle of the century. In North America, the trends are similar. In 2050, according to United Nations projections, it is possible that nearly as many babies will be born in the United States as in China. Indeed, the population of the world’s current demographic colossus will be shrinking. And China is but one particularly sharp example of a widespread fall in birthrates that is occurring across most of the developing world, including much of Asia, Latin America, and the Middle East. The one glaring exception to this trend is sub-Saharan Africa, which by the end of this century may be home to one- third of the human race.
... Perhaps the most striking fact about the demographic transformation now unfolding is that it is going to make the world look a lot more like Europe. The world is aging in an unprecedented way. A milepost in this process came in 1998, when for the first time the number of people in the developed world over the age of 60 outnumbered those below the age of 15. By 2047, the world as a whole will reach the same point.
The world’s median age is 28 today, and it is expected to reach 38 by the middle of the century. In the United States, the median age at that point will be a young ish 41, while it will be over 50 in Japan and 47 in Europe. The United States will be the only Western country to have been in the top 10 largest countries in terms of population size in both 1950 and 2050. Russia, Japan, Germany, Britain, and Italy were all demographic titans in the middle of the 20th century. Today, only Russia and Japan still (barely) make the top 10. They will not stay there long. The world has changed. There is more and faster change to come.】