the US, it's not as bad because of immigration, but if you only include native born americans ( non-immigrants ), our population demographics is even worse than japan. Native born americans stopped procreating at replacement levels since the late 1960s.

Unless Japan/Western Europe bring in millions of immigrants, they are going to be in for major problems because of their liabilities to retirees. A shrinking population can't hope to support the older generation. And the worst part is that if younger generation has to support the older generation, they will put off have kids because they simply can't afford them. Which in turn exacerbates the problem and it turns into a vicious cycle.

But immigration also has its own problems. So the industrialized world is caught between a rock and a hard place.

Even in the US, the pro-immigration policy is a stop-gap measure. The population can't grow forever.

Blackrock has an interesting paper on the overall trends.

https://www.blackrock.com/investing/retirement/blackrock-retirement-institute/workforce-and-economics/challenges-and-opportunities-in-an-aging-population