The speed at which Japan's population is aging is mainly a result of its rapid economic development, which is common to many other East Asian countries. Two aspects of the aging of the population are an increasing share of the elderly and a declining labor force. The larger the number of elderly, the more the transfer of income between generations, and the fiscal burden rises. The shrinking workforce will lower the economic growth directly, and indirectly through the falling saving ratio. 

Japan aging population focus on: https://www.sciencedirect.com/science/article/pii/S1049007897900191

The brith rate went lower in Japan.