"Around the world, death rates gradually decreased in the late 19th and the 20th centuries, with death rates in the developing world plummeting after World War II thanks to the spread of modern medicine. In much of the developing world the decline in death rates preceded the decline in birth rates by 20 years or more, resulting in record-high rates of population growth of 3 percent or even 4 percent a year. Since the 1960s birth rates have also been declining rapidly in most developing countries except those in Sub-Saharan Africa and the Middle East. This trend in birth rates in the developing world is comparable to what took place in Europe and the United States in the 19th century."
The world's population is growing—and aging. Very low birth rates in developed countries, coupled with birth rate declines in most developing countries, are projected to increase the population ages 65 and over to the point in 2050 when it will be 2.5 times that of the population ages 0-4. In 1950, there were 335 million children in the 0-4 age group and just 131 million people ages 65+. According to United Nations Population Division estimates for mid-2010, there were 642 million persons ages 0-4 and 523 million ages 65+. That is, of course, quite a change. The UN also projects that for the first time in history, the 0-4 age group will decline between 2015 and 2020, having peaked at around 650 million. The 65+ population is projected to exceed the 0-4 population during that same five-year period, rising from 601 million in 2015 to 714 million in 2020, although at the time that happens will depend on how fast birth rates in developing countries decline.