Figures from a census released on Tuesday show that China faces a demographic crisis that could stunt growth in the country, the world’s second-largest economy. China has long relied on an expanding and ambitious work force to run its factories and achieve Beijing’s dreams of building a global superpower and industrial giant. An aging, slow-growing population — one that could even begin to shrink in the coming years — threatens that dynamic. China’s aging-related challenges are similar to those of developed countries like the United States. But its households live on much lower incomes on average than in the United States and elsewhere. In other words, China is growing old without first having grown rich. “Aging has become a basic national condition of China for a period of time to come,” Ning Jizhe, the head of China’s National Bureau of Statistics, said at a news conference announcing the results of a once-a-decade census.