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.
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.