The biggest problem facing the world isn’t that birth rates in developed countries are low. The real problem is that birth rates in the world’s worst societies remain sky high by comparison.
The biggest problem facing the growing preponderance of elderly people in developed countries is the very low (for a couple of generations) fertility in developed countries. Japan doesn’t have elderly people committing crimes so they can be cared for in jail - as opposed to not at all - because Niger has a high birthrate.