Right now, the world is evenly divided between those under 28 and those over 28. By midcentury, the median age will have risen to 40. ... By 2018, 65-year-olds, for example, will outnumber those under 5 — a historic first. In 2050, developed countries are on track to have half as many people under 15 as they do over 60. In short, the age mix of the world is turning upside down and at unprecedented rates.
This means profound change in nearly every important relationship we have — as family members, neighbors, citizens of nations and the world.
The reasons include longer life expectancies in developed countries and lower birth rates.
(One of the key transitions in U.S. demographics has to do with different birth rates between residents born here and recent immigrants to the country, and that same trend is also keeping the country "younger" than some other developed nations.)