Final answer:
The elderly population is increasing most notably in high-income nations, with a significant shift already taking place and expected to continue through 2020 and beyond.
Step-by-step explanation:
The share of the elderly population is increasing most quickly in high-income nations. Many of these nations are already experiencing a significant growth in their elderly demographic and are expected to have a much larger share of the elderly in the near future, around the year 2020. While low-income countries currently have a higher proportion of youth and young adults, they are projected to see a dramatic increase in their elderly populations by about 2050, which will also have profound effects on their societies. Nevertheless, in the current timeframe, the increase in the elderly population is more pronounced in high-income nations.