By 2050, China’s elderly will account for 26% of it’s population, the US has massive debt with only 13% of it’s population beyond working age.
http://www.economist.com/node/21553056
http://www.investopedia.com/articles/investing/032013/us-vs-china-battle-be-largest-economy-world.asp
The United States has been the world’s biggest economy since 1871, but that top ranking is now under threat from China. The Asian giant has achieved economic growth averaging 10% since it initiated market reforms in 1978 and, in the process, lifting almost half of its 1.3 billion population out of poverty and becoming the undisputed second-largest economy. China’s gross domestic product (GDP) – in terms of current prices and market exchange rates – was estimated by the IMF at approximately $8.25 trillion in 2012, which is just over 50% of US GDP of approximately $16 trillion for the same year. While that is a significant gap that may take China many years to close, using another measure known as Purchasing Power Parity (PPP), China is forecast to race past the U.S. in just a few more years.