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.

I can imagine one in theory, but not in practice without, at a minimum, a very painful transition that would need to occur in the near future.