Just saw this great chart from McKinsey (click to enlarge):
I didn't know Singapore's median age was into the 40s! Nor that America's median age was well above China's. It's not obvious to me from casual observation. As a friend once asked me, "Why do people say that Singapore has an aging population? I see plenty of children on the street with their parents." In comparison, one rarely sees children in Japan. But the numbers tell a different story from the perception.
One other interesting point to note is the way the authors have classified the countries. Most countries are classified by their levels of education and age of population. India and China, though, get their own independent categories! That speaks for itself. Another way to do it would have been to separate out urban India and urban China, and lump them in with other parts of the world with similar characteristics. Urban China and India are older and wealthier than the rest of each country, so this could substantially alter the picture.
This chart is from a longer McKinsey report called The World at Work, which is available for free download.
Another great source of this sort of data, visualised just as nicely as this, is the absolutely spectacular website Gapminder. If you're reading this blog, you're probably aware of this website already.