Most people are aware of how declining fertility rates, and longer life spans, means that demographics of advanced economies is trending towards more older people. Japan is the common example of where it is currently a problem, but it will be a global issue to content with later this century.
A figure from a study published in Lancet really drives it home:
The number of people aged 80 and over, globally will increase from the present 141 million to an extraordinary 866 million in 2100.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30677-2/fulltext
In a nutshell that means that younger people will have to work more to support the older people, directly or indirectly.
It also means that capitalist countries will have a system that doesn’t work with a decreasing population, and they population will have fewer workers and more people who don’t spend much money.