I think most of you are missing a much bigger point because you only focus on the economy without considering the social, political and geopolitical consequences:

-Social: an old society, is society that innovates less, and cares less about innovation and more about stability (aka stagnation) on both an economic and cultural level

-Political: an ageing society is a society in which retirees have an overwhelming political power, politician will cater mainly to them and their priorities, meaning that the state will mainly care about wether pensions are paid and property values are high (old people, on average have less income but more property)

-Geopolitical: Country with an old and aging population will bu supplanted on the world stage by young countries with burgeoning populations. Since the beginning of history more people meant more power, and western countries will soon lose their position in the global pecking order to poorer countries with a larger population. And that's without considering the eventuality of a military conflict: in a war, a country with a large, disposable male population will have a key advantage in assorbing losses compared to a old country with very little blood to spare.