The aging of a population structure is an inevitable path of demographic transition, as an inescapable result of decline in fertility rate and extension in life expectancy. Although demographic transition occurred earlier in Japan than in China, the two countries had similar patterns, both of which took a much shorter period of time than Western countries to complete demographic transition, as well as have been aging at a rapid speed that has rarely been seen in the world. Japan has the highest level of population aging in the world, and China has been experiencing a very fast pace of the population aging process and has the largest older population. Drawing upon data from World Population Prospects (2019), this paper compares changes in population aging in both China and Japan. Findings show that Japan's aging process is 30 years ahead of China, but China has been changing in a similar way as Japan. To be specific, both countries experienced four phases of the population aging process: accelerated development period, rapid development period, slow down period and high-level maintained period. In addition, both countries had a quick growing rate of population aging. It will take China 23 years and 10 years respectively for the aging rate increasing from 7% to 14% and then to 20%, while Japan took 24 and 11 years respectively, which is much shorter than developed countries in the West. Furthermore, China has a faster pace than Japan in population aging in the next 25 years. We found that from 2019-2044, China's aging rate, elderly dependency ratio, oldest-old coefficient and median age of population will increase 13.24 percent points, 24.21 percent points, 8.33 percent points, and 8.47 years, while the four indicators of Japan will increase 8.38 percent points, 22.52 percent points, 8.29 percent points, and 6.20 years, respectively.
