World's 65 and older population to triple by 2050

The world's 65-and-older population will triple by mid-century to 1 in 6 people, leaving the U.S. and other nations struggling to support the elderly.

The number of senior citizens has already jumped 23 percent since 2000 to 516 million, according to census estimates released on Tuesday. That's more than double the growth rate for the general population.

The world's population has been graying for many years due to declining births and medical advances that have extended life spans. As the fastest-growing age group, seniors now comprise just under 8 percent of the world's 6.8 billion people. But demographers warn the biggest shift is yet to come. They cite a coming wave of retirements from baby boomers and China's Red Guard generation that will shrink pensions and add to rising health care costs.

Germany, Italy, Japan and Monaco have the most senior citizens, with 20 percent or more of their people 65 and older.

In the U.S., residents who are 65 and older currently make up 13 percent of the population, but that will double to 88.5 million by mid-century. In two years, the oldest of the baby boomers will start turning 65. The baby boomer bulge will continue padding the senior population year after year, growing to 1 in 5 U.S. residents by 2030.