The key determinant (@ Kennedy) is relative, not absolute decline). British GDP grew, in absolute terms, while it declined relative to the US and others. Thus do great powers ebb...
The US peaked in early 2000, though a debt bubble managed to keep things aloft for another few years.
http://www.marketwatch.com/news/story/Chinas-GDP-overtake-US-early/story.aspx?guid=%7BF82D6566%2DB725%2D4D5F%2DBAA7%2D30F0E5BC7AD3%7D
When China meets the US, China will be poorer on a per capita basis, but will FEEL richer as it is ascending; while the US, with a small population, will be richer on a per capita basis, but will FEEL poorer, as it is descending.
The markets have already started to discount this process -- as markets anticipate....