1. directly: in a democracy where the majority of voters are 60+ years old and retired, politicians will end up seeing the old and retired as their base and pandering to them; and this will lead to them implementing policies that serve the old and retired, to the detriment of those of working age. (See: literally anything about Japan's work culture.)
2. indirectly: social mores evolve over time, but this mostly occurs within the more-flexible, still-identity-forming minds of people of young ages. You can think of "modern" social mores as viral memes that spread easily between the "young" nodes in a social network; but which won't spread to/through the "old" nodes of the network. A a "top-heavy" society, where the old greatly outnumber the young, doesn't have the mean virality coefficient necessary to allow "modern" social mores to spread and achieve fixation. This in turn means that younger generations will feel they are living in a society that is "stuck in the past", with "outdated" thinking, where they cannot affect meaningful social change, despite everyone in their own (usually generational-cohort) social circles agreeing with them.
As a fact of nature, young adults must do the hard labor necessary to keep the world operating. Historically, there have always been more of them than there were people physically incapable of doing that work: the old and very young.
If there are too many old people and not enough young people, who will take care of the old people?
If population is declining, it breaks how these programs are funding because there’ll be less n the younger generation to pay for the more people in the older generation.