putting the problem in terms of economics isn't incorrect, but I think it can be framed in a slightly more human and horrifying way:

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?