China's decline would be massive.
These projections are more accurate than one would imagine, and the early part of these trends are more or less baked-in. Though they obviously aren't perfect. The scenarios have certain underlying assumptions. They can't predict new disruptive events like say the HIV crisis. And they are generally more inaccurate for developing countries where the base statistics may be less reliable.
Africa isn't shown here, but it is projected to have a working age population that is equal to Asia's by the end of the century.
Something that helps the US here is our relative openness to immigration. Conversely China and Japan are not as open to migrants.
Even still Japan for the last few decades is a good example of how significant demographic challenges don't necessarily mean economic catastrophe. But this should be an interesting century.