Final answer:
Japan is the most rapidly aging country in the developed world.
Step-by-step explanation:
According to the text, the most rapidly aging country in the developed world is Japan. It states that many high-income nations are approaching a situation where the elderly will form a larger share of the population, and Japan is particularly affected by this issue. The demographic changes associated with population aging will have a significant impact on the standard of living for both the young and the old.