according to https://en.wikipedia.org/wiki/List_of_sovereign_states_and_dependent_territories_by_fertility_rate  Japan's fertility rate is 1.4, really low.  You have to maintain a fertility rate over 2 to maintain a culture.  At 1.3 a population would be cut in half in 45 years an in 100 years would be down by 75%.  No kids means decline in population, plain and simple, people die every day.