Irrelevant. There's no link between population decline and startup culture. There are still millions of young people growing up over there. There are multiple reasons not to go to the US, with language and atrocious visas at the top of the list.
Unlike the US, there does seem to be a recognition that people are part of the economy of the country and that there are mutual shared interests in making things work. The issue seems to be more in the speed and adeptness at which changes are made.
That being said, the biggest obstacle to start-ups in Japan is the Japanese aversion to risk. The most talented people would rather work for peanuts in large Corporations or in secure Government jobs rather then risk failure in a start-up.
There seems to be a lot of "don't give old people this" whenever the question of age-based rules comes up.