When Tokyo hosts the 2020 Summer Olympic Games, the city will be the most populous it has ever been. It will also be the most populous it will ever be. The 2020 Olympics will mark the peak of Tokyo’s population before it goes into steep decline.
According to the Tokyo metropolitan government, Tokyo’s population will reach 13.36 million people in 2020, a rise of 200,000 from 2010.  However, over the forty years following 2020, the population will drop by 20 per cent to 10.36 in 2060 (roughly the same population as it was when Tokyo hosted its first Olympics in 1964).
Not only will Tokyo’s population be smaller, it will also be older. The number of people aged 65 or older was 2.65 million in 2010 (making up 20% of Tokyo’s total population). In 2060, those figures will be 4.07 million (39%).  Those in the “production age population” of 15-64 year olds will decline in the same period from 8.85 million (67.3%) to 5.53 million (53.4%). Or, to view it another way:

“…3.3 persons in the production age population supported one elderly person in 2010. In 2060, the figure will drop to 1.4 persons.” Tokyo’s metropolitan government is certainly not blind to the challenges that an ageing, declining population will bring.  It is compiling a “long-term vision” document to set out policies for the next 10 years, including urban planning, child-rearing support and nursing care services. 
Tokyo is of course merely reflecting Japan’s future. As the Telegraph reported a few months ago, Japan’s population declined by almost 300,000 people last year and is projected to decline by over 25% by 2050. So actually Tokyo is expected to decline less quickly than Japan as a whole, presumably because it will continue to attract internal migration from rural areas. With this sort of societal malaise in Japan, I think that the Tokyo metropolitan government has a challenge that not even Sisyphus would envy.