An aging population doesn’t work or save. It doesnt consume as much either. It isn’t a source of innovation or investment capital. It doesn’t pay as much in taxes, so necessary government activity will be constrained. Capital shortages will drive up interest rates. How’s all that for a start?
“An aging population doesn’t work or save.”
You are describing problems that arise when there are oscillations in birth rates.
For example, you can have a lot of old people with few young to run the economy that existed when the old people were young.
A balanced population will do well regardless of its size.