Because old people tend to have more wealth since they had a lifetime to accumulate. This places with older people tend to be richer, but that does not necessarily translate to what happens to an average person in a society.

For example, in the US the median network for an 69 year old (right around retirement) is over 33 times as much as an 18 year old [1], with values in between spread as expected. So a society with significantly more old people (such as Japan) would have higher median wealth but not necessarily more income (for example, US pays more than Japan on average [2], but we're younger).

So, if you move to Japan from the US assuming you'll end up richer, you may be making a mistake because you made that choice based on comparing things incorrectly.

This is why it matters.

[1] https://dqydj.com/net-worth-by-age-calculator-united-states/

[2] https://countryeconomy.com/countries/compare/japan/usa?sc=XE...