low fertility rate
Explanation: Japan is a country that has been having problems with negative demographic trends for several decades now. The main reason behind this is that the newer generations tend to have very low birthrates. Big portion of the younger people delay getting married until they are in their late 30's because of their careers or financial reasons, and when they do, they tend to delay having a child, so they only have time to have one, rarely two. Also, there is lot of younger people that do not want to have children, or do not want to get married. Considering that Japan is also one of the top three countries when it comes to life expectancy, it has a population that is aging very quickly, creating more and more problems as the time passes on.