The world's 65-and-older population will triple by mid-century to make up 1 in 6 people.

Census estimates released Tuesday show the number of senior citizens has already increased 23 percent since 2000 to 516 million, more than double the growth rate for the general population. The fastest-growing age group, seniors now comprise just under 8 percent of the world's 6.8 billion people.

By 2050, the senior group will increase to 1.53 billion.

Demographers attribute increases to declining births as well as a coming wave of retiring baby boomers. They warn that the U.S., China and other countries could face fiscal crises in the coming years as governments struggle to pay for health care and pensions of their rapidly aging populations.