Based mostly on this article on Japan. What is important when looking at dependency ratios is not the ratio of old persons to the working age population but rather the ratio of non-working citizens to working ones. And this includes people (usually) younger than 24, especially in developed countries (where most people attend some type of college).
When examined under this perspective the relevant picture is quite different: Non-working dependency ratios were extremely high during capitalism’s ‘Golden Age’ (1950 – 1970). Greece will hit ratios last seen in 1975 at.. 2040 (with the dependency ratio being much higher in the ’50s) while Germany will only get into trouble after 2025-2030.
As long as pensions are not a ‘defined benefits’ but rather a ‘defined contributions’ scheme and remain flexible, population aging will not lead to the ‘dooms day’ scenarios that some people fear about. We managed to take care of baby boomers just fine during capitalism’s finest hour.