The correct answer is - aging population.
The aging population phenomena is becoming a big concern in the past few decades in some parts of the world. The most affected by this trend are the European countries, as well as Japan, the United States, Canada, Australia.
As the countries get more developed, and people earn more, it seems that they have much less children, usually one or two, and many people don't even have a child, and considering the upwards trajectory of the life expectancy, it results in an ever growing old population, and smaller and smaller young population.
This is a very dangerous trend on the long term as a nation may simply die out over time, as well as creating big economic issues.