So, here's a rough summary of what I think I've figured out for myself (your mileage may vary) while I've been writing this series so far.
Education is by definition "the development of the right use of reason and the right use of freedom." TEducation necessarily includes moral and theological understanding, the transmission of culture, and an especially firm grounding in human sexuality. A detailed list of features of education is here.
Education includes features like skills-training and home economics. I believe this is supported by Catholic doctrine: an individual who will likely marry must be educated so that, by means of that education, he can "establish a family in favorable moral, social, and economic circumstances."
When it comes to determining if your education is adequate, "latent competency" counts. I wrote about latent competency here. Basically, if you've learned how to learn, and you possess the intrinsic motivation to learn, then education has done its job. You'll acquire what you need to know when you need it.
The parts of a liberal-arts education that are truly necessary for full human development must be included before high school graduation. When young people are legally emancipated at an arbitrary age, parents have a positive duty to present and develop (at least to latent competency) all the necessary components of a whole education by then; or to cultivate successfully in their children the values that will motivate their children to go on to acquire those competencies at older ages. Otherwise they may fail in their task of education, when a legally-adult son or daughter abandons it unfinished.
Parents do have a positive duty to support their sons' and daughters' education beyond age 18 if they haven't acquired at least "latent" ability to establish a family in favorable circumstances. I believe it's contrary to Catholic understanding of the purpose of education to declare that parents can morally declare children "on their own" at an age that's derived from the civil government's arbitrary definition of the age of majority. But this "latent ability" can be quite broadly defined. A 19-year-old who's secured a scholarship and has a prudent plan for her education and future employment, and is possessed of character traits that are maturing towards healthy adulthood, can be assumed to have it, only because she's on a track that is likely to be successful.
Parents also have a positive duty to make decisions concerning the good of the whole family, including their own needs (present and anticipated). This creates a tension between what a single individual might demand of the parents and what parents ought prudently offer.
Adult sons and daughters who are so supported by their parents retain the moral duty of filial obedience. This extends to all matters decided by the parents for the good of the family, and continues until they become "emancipated" and are living outside the parents' home. Pragmatically speaking, this means that parents rightly place judiciously chosen conditions upon financial support of their offspring's continuing education. The parents choose these conditions to the end of helping their son or daughter acquire the ability to "establish a family in favorable moral, social, and economic conditions," within the limits set by the good of the whole family.
Parents may not pressure their sons and daughters in the choice of vocation, profession, or spouse. But I think this still allows them to prudently offer different kinds of support to different children, tailored to the young person's particular plans, abilities, and circumstances. This is so even if this creates the appearance of offering more money to support some choices than others. Some choices need more money; some choices might suck money into a black hole, never to be seen again. Some young people might achieve maturity with a little more hand-holding and direct support; some young people might need to learn from the experience of trying to support themselves. Education is not one-size-fits-all.
Though parents have the duty to begin education and carry it to a certain point, the adult learner bears the responsibility for a great deal of his education. I divided the tasks up here. Certain professional specializations; self-education through choice of leisure; retaining an understanding of the whole human person; cultivating appropriate patriotism; all these are assigned not to the parents, but to the adult. They tend to be what you might classify as "lifelong learning."
Large amounts of education debt are positively to be avoided. Debt is anti-vocational. There are probably a very few circumstances in which it is warranted, but always with caution.
The old assumptions about the value of college don't hold firm anymore. While in the aggregate college graduates earn more and suffer less unemployment, it's not clear that a given individual will significantly raise his future earnings (enough to justify the cost) by enrolling in a four-year university. Different degree programs offer widely varying prospects. The "signal" of a quality employee that used to be conferred by any college degree is getting diluted by sheer numbers. Particular individuals might do better economically if they choose an alternative to four-year-college-right-out-of-high-school; for other individuals, choosing a different path might be morally safer or might more securely develop a mature character. Finally, the quality of the education (measured in the richness and quality of the culture it promotes; the variety and vividness of the intellectual atmosphere; the commitment to authentic human values; the challenge to an engaged student) varies from institution to institution, department to department, so that many students graduate seemingly intellectually unchanged.
It makes sense to delay spending any money at all on postsecondary education until the young person has a plan that is ordered to establishing a family in favorable moral, social, and economic circumstances. Without a plan, the education cannot be tailored to the vocation, and the money spent has a high probability of returning low value, either in tangibles or in intangibles. The delay can be spent in productive work, and if that's not much fun, then it is itself a motivation to come up with a plan. While you're at it, it's a good idea to have an "escape hatch" plan in case the first one becomes unfeasible. As Darwin said, "Borrowing tens of thousands of dollars is a bad way to avoid making decisions;" in my opinion, from the point of view of parents, "spending tens of thousands of dollars is a bad way to enable someone else to avoid making decisions."
Just as young people ought to consider alternative means of self-development, parents ought to consider alternative ways of materially supporting young people. I outlined some thoughts about that here.
Consider whether you'll actually get the four things you might be buying when you pay for college: a signal of quality; a required credential; a set of vocational skills; and a difficult-to-measure acquisition of other experience, skills, and knowledge. Will you be paying too much for a poor signal, or will you be "found out" for flying a false-flag signal (because you're really not the kind of person that employers are looking for in a college graduate?) Do you have what it takes to obtain the required credential, or will you pay a lot of money and then drop out before getting there? Will the education really develop the skills you are hoping for, or is it a bunch of empty promises? How much are you willing to pay for returns that you think you'll value, but that are unmeasurable?
I think that's a good summary. I'm going to shift gears in my next post and consider... The Case Where Money is No Object.