Even if it starts stagnating it will pass the US in total GDP. China has 10x the population of Japan. Not that it matters. The average Chinese will still be a lot poorer than the average American.
However, thanks to the PRC's 1979 "one-child policy", they look to be the first country to skip from the "industrializing" stage which generally has reasonable population growth (weasel wording since Japan's fertility started declining in 1974 and has yet to stop), to the sort of modern social welfare state we see in the West with too few producers trying to support too many dependents. Except of course they don't have much of a welfare state.
It's sometimes called the 4-2-1 problem: 4 grandparents produced 2 parents who produced 1 child. It's one of the reasons for their very high savings rates, even with negative real returns. And like Japan, the retirees are in for unpleasant times when they try to turn paper wealth into real consumption.
