Time and again, I've written that it is a mistake to write America off as we contemplate China's recent rise and judge whether (or even when) China passes us by as the dominant power. We retain basic advantages that should cushion our relative decline, our geography makes us the key balancer even if we sink to second place, and China could falter--or even splinter.
This summary of a chapter that looks at the demographics of the big Pacific players (Russia, China, India, Japan, and America) and concludes that Russia faces huge problems, China and Japan face problems (slower growth or decline, respectively), India should be better off on balance, and America--wait for it--should gain. The time frame for the comparison is 2040.
I have my own projections--purely a WAG--here, if you are interested.