Beijing ? The principal’s office is now the mah-jongg room. Classrooms have become bedrooms equipped with bells to summon the nurses. And the clamor of more than 1,000 elementary school students has faded into the shuffle of slippers through the halls.

Four decades after opening its doors in Beijing, the Dongzhimen Elementary School reinvented itself last year as the Three Harmonies Senior Citizens Home. The transformation reflects a vast and sobering challenge on China’s horizon.

“There are very few young students anymore, but more and more elderly people,” said Three Harmonies’ manager Wang Shuyuan.

A generation after China adopted its unprecedented one-child policy, the world’s most populous nation is aging faster than any major country in history. The graying of the population, lost in the astonishing statistics on China’s economy, threatens to hinder future growth and strain a frayed public-welfare system.

“They are looking at 400 million old people, 30 years from now, the vast majority of whom will not have pensions or health care or extended family,” said Richard Jackson, director of the Global Aging Initiative at the Washington-based Center for Strategic and International Studies. “This is social and political dynamite, and the government knows it.”

The problem is a peculiar side effect of progress. For most of Chinese history, people older than 60 rarely numbered more than 7 out of every 100 people. But improved health care, sanitation and living standards since the Communist Revolution have allowed the average person to live more than 30 years longer than in 1949. At the same time, China has restricted family size since the late 1970s in an attempt to control population growth.

The result is a China-size version of America’s Social Security crunch, in which there are neither enough offspring nor pension funds to finance tomorrow’s retirements. But China faces an even greater hurdle, because its per capita income remains barely a tenth of U.S. levels. As economists put it, China is getting old before it has gotten rich.

“Feeding the people is the most common problem in developing countries, and taking care of the elderly is the most common problem in developed countries. China has to solve both at the same time,” said Hu Angang, an economist at Qinghua University in Beijing.

Like most developing countries, China remains relatively young; those older than 60 make up just 11 percent of its 1.3 billion people. But things are changing fast. By 2050, those over 60 will make up 31 percent of the population – higher even than the U.S. Short of a major change in the one-child policy, China will take just 25 years to age as much as Europe did during the last century, U.N. statistics show.