Simran Khosla designed this nice map to compare median age across the globe. You may see it in more details in each continent, here. Simran has also done a similar map comparing Life Expectancy at Birth.
And a few papers and links on population aging.
- Map of the Day: Where Seniors Are Moving to in the US
- Is low fertility really a problem? Population aging, dependency, and consumption (ht Bernardo Queiroz)
- The pace of aging: Intrinsic time scales in demography
- Handbook of Sociology of Aging
- Population Change among the Elderly: International Patterns
- The dependence burdens of aging babyboomers: a cross-national view