If you read the latest Charles Goodhart's book ageing population would bring period of permanent high inflation. And during high inflation periods trend following (macro) strategies work well [I don't have a link to the paper at the moment]... So this should be rather simple...
Japan seems to be an exception to the rule, there are probably other factors that resulted in deflation during falling population era [I am fan of Richard C. Koo here - 2].
However it is not established yet - there is active discussion going on the topic - falling populations vs inflation/deflation. It is quite interesting actually.
Try Dimitri Kofinas's discussion with Charles Goodhart in this podcast [3] if you can spare an hour...
[1] https://www.amazon.com/Great-Demographic-Reversal-Societies-...
[2] https://www.amazon.com/Holy-Grail-Macroeconomics-Lessons-Rec...
[3] https://hiddenforces.io/podcasts/charles-goodhart-demographi...
The great thing about demographics is it's about the only area in which we can make predictions about the (near) future with any degree of precision (or accuracy).
However pointing out these looming problems and advocating family-friendly and child-bearing-friendly polices just got Goodhart labeled as a right-wing anti-gay nutjob back in the '00s and '10s.
1. Goodhart has finally (2020) written a book after decades of writing pieces on Project Syndicate and VoxEU: https://www.amazon.com/Great-Demographic-Reversal-Societies-...