American capitalists and economic planners fret about "Japan Syndrome". To have more productivity and more consumption i.e. GDP growth, you need more people as a core driver. We don't actually need this, we could do fine with a stable population, but capitalism needs to grow or perish.
Declining populations are trickier for most economic concepts though. Less labor, less consumption. That said, a slight decline can leave more houses unoccupied which can be good. A major decline would mean so many unoccupied houses that you would have broken and abandoned houses though because it would be too costly to deal with the abandoned units.
If you or anyone you care about is or will be elderly and is not financially independent, you should care.
This has nothing to do with capitalism; it's a resource allocation problem. We spend inordinate amounts of money on end of life care, and any changes are currently unacceptable to voters.
You're talking about age structure, but overall population age-structure can be adjusted by immigration flows, births, deaths, etc. My point was about the total population number.
You can imagine a steady-state population where the age structure is stable and productivity is high enough to sustain the retirees, trainees, and disabled.
Declining populations are trickier for most economic concepts though. Less labor, less consumption. That said, a slight decline can leave more houses unoccupied which can be good. A major decline would mean so many unoccupied houses that you would have broken and abandoned houses though because it would be too costly to deal with the abandoned units.