Up to 1950, Japanese population was under 90 million. I was there in 1950, and the place seemed just right regarding population, ability to feed itself after the war, and beautiful countryside.
A few years ago the Japanese produced a film called something like The Mountain of Nagoyama, which told about life in a medieval village. The village kept itself off starvation by a rule (among other strategies) that people over sixty went up the mountain to die. Everyone, including the elderly, accepted this. It fitted the Japanese willingness to sacrifice for the good of everyone else, which is also seen in the kamikazi suicide flying in the 2nd world war.
The Japanese are very pragmatic, and have a history of self-sacrifice and stoicism. Their attitude to death is not that of the West.
They may well solve their ageing population not by increasing the total population with more young people – a growth policy which must reach disaster point at some time – but by decreasing the numbers of the elderly.
This could be by the voluntary deaths of the demented and painfully-dying, two very costly groups. The Japanese people could very well be wiling to do this for themselves – (and bring the numbers of adult nappies down to the numbers of child nappies.)