That's true for most of the developed world though. And honestly that's what's actually required because the planet can't sustain this level of consumption from this many people so the population actually does need to go down.
Completely agree about the birthrate needing to decline, however Japan is in a worse position due to Japan's fertility rate stands at 1.3, while the United States is at 1.6. US also some what plugs the gap with significantly higher legal and illegal immigration than Japan. Japanese tend to live longer as well. So they are ending up more lopsided than the US when it comes to age spread.
You would want a slow decline though rather then a total plummet, otherwise you’ll end up with a bunch of older folks and not enough younger ones to support them.
It's cheaper to care for elderly than pay for raising children. Eventually the elderly die and those resources are freed up. But with children, you end up with adults using even more resources.
A huge factor for the Rennaissance was all the elderly dying in the Black Plague. Less people is better for workers because their labor becomes more valuable. It's only the owners that suffer in population decline because their servants are more expensive.
The children turned adults will generally be working and contributing to the economy though. Elderly will not and use up a metric ton of healthcare resources.