China may not pass the U.S. at all, much less in the next 10 or 15 years. It is a fundamental mistake to graft the previous 30-year trend onto the next 30. The China of 1949 to 1978 looked nothing like the China of 1979 to 2009, and there are powerful reasons to believe that 2010 to 2040 will be very different again.
The most important and most certain aspect is an aging Chinese population. The last 30 years were characterized by demographic expansion very favorable to growth in the PRC. The next 30 will be generally characterized by demographics unfavorable to growth. The transition from growth-conducive to growth-hostile demographics will begin about the middle of this decade and continue indefinitely.
In 1985, 15-year-olds to 29-year-olds made up 47 percent of the working-age population in China. In 2030, they will make up only 26 percent. By 2035, a daunting 280 million people are projected to be 65 or older. Far fewer people will be working to support far more retirees.
The previous economic challenger to the U.S., Japan, has faced the same problem. Over the past 15 years, the Japanese economy has not grown at all. Over the next 15 years and beyond, China's population structure will become more and more similar to Japan's today.