Washington ? 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 because of 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.

“The 2020s for most of the developed world will be an era of fiscal crisis, with a real long-term stagnation in economic growth and ugly political battles over old-age benefits cuts,” said Richard Jackson, director of the Global Aging Initiative at the Washington-based Center for Strategic and International Studies.

“In emerging countries like China, they will face the real prospect of a humanitarian aging crisis,” he said.

China’s current ratio of 16 elderly people per 100 workers is set to double by 2025, then double again to 61 by 2050, partly because of family planning policies that limit most families to a single child, Jackson said. Without a universal pension system to cover all elderly, millions of older Chinese could fall into poverty, creating social and political unrest and shock waves that could ripple through the global economy given the country’s economic heft.

In the U.S., immigration of younger residents has helped slow aging of the total population. Still, Medicare is projected to become insolvent by 2017, and President Barack Obama has said that overhauling Social Security and Medicare is critical. In making reforms, Obama and a Democratic-controlled Congress risk alienating a 65-and-older voting group by cutting benefits or their younger generation by raising payroll taxes.

“As they age, boomer support on issues like Medicare and retirement security will be just as key for continued Democratic success as the party’s hold on younger minority voters,” said William H. Frey, a demographer at Brookings Institution, citing higher voting rates among seniors who could prove important in key states such as Ohio, Pennsylvania and Missouri.

The Census Bureau updates projections each year on a variety of global demographic trends, including fertility and mortality rates and life expectancy. Its estimates are separate from figures released by the United Nations Population Division.