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. This is an exact reversal of the situation in 1950.
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 precisely when that happens will depend on how fast birth rates in developing countries decline.
More information about these trends can be found on the Population Reference Bureau  website.
Source: Population Reference Bureau