To say that Japan's population will shrink over the next 83 years is an understatement. According to official state figures, the country will go from about 126 million people today to about 50 million in 2100, a 60% plunge.
Moreover, the makeup of Japan's population will utterly change, too. From about 15% of the population, people 65 and older will be 35% in 2100. And the working age population whose salaries are supporting the old will plunge: in 1970, Japan had 8.5 workers to support every retired person; in 2050, the number will be 1.2.
As you see in the chart below, these numbers reflect the trend in most of the world. Right now, the median age across the planet is around 29. In 2100, it will be 42. When you exclude Africa, the whole world will be, on average, 60 or older in just over three decades.