Japan is one of the oldest populations on the planet, getting older. Just about everything in how society runs (in Japan or any other developed economy) assumes that old people get to retire, taken care of by young people. But, this requires a minimum ratio of young to old. Eventually, and Japan may already be getting there, you are putting too much strain on the economy of young people, so they cannot get enough resources to have children of their own, and you enter a downward spiral. You either end up with old people having to work past the time when they can really do so, or old people facing a declining standard of living as there is not enough working population to take care of them.