Practically speaking, it should be a cultural imperative that heterosexual young people be married between the ages of 18-25, and that they not only be able to support themselves, but several children as well, *at that age*.
This takes several axiomatic changes to our society, however.
1) Boy and girl children need extensive chaperoned cross-gender socialization growing up. Today, they get almost none, and it leaves them socially isolated and unable to communicate or cooperate with the other gender when they are adults. Yet they are expected to magically find a mate and marry them, at where? Bars?
2) Society used to recognize the essential importance of marriage and procreation, and how it needs special encouragement in the law, especially favorable tax and housing law. Children are inherently expensive. Government needs to provide greater support, but it must also discriminate against those who are not married and have no children at the same time.
A good way of doing this is to provide tax incentives to businesses in hiring married young people, and even more incentives if they have children, resulting in higher pay and benefits than would be typical for entry level jobs.
3) There is also a strong need to create cultural alternatives to those who should not or decide to not have children. Comparatively, in old China, respectable women whose families could not afford dowry, or who otherwise didn’t marry, could become ‘Amah’, adopted by other families to provide some light services in exchange for a stipend, but as respectable women, not servants or prostitutes.
This is a very alien idea to the US, but is essential to keep such people “out of the way” of married, breeding couples. It is a socially acceptable alternative to marriage and children.
You have made some excellent points, especially (1). Our society no longer supports marriage for young people, then wonders about the declining fertility rate and eroding tax base (no more taxes from households = property owners).