The rapid aging of Japan's population after World War II was mainly influenced by a significant increase in life expectancy and a decrease in birth rates. The decline in fertility had a bigger impact on population aging than the decline in mortality, which was more pronounced in Japan compared to European countries. The study projected population trends up to 2082 and found that the combination of lower birth rates and longer life spans led to a substantial increase in the proportion of elderly people in Japan. This rapid aging was largely driven by the decline in fertility rates, but the decrease in mortality rates also played a significant role, especially in middle and old age groups.