Cynicism alert. The purpose of widespread college education is socioeconomic. People simply wouldn't throw so much money and time after this game (starting, in some places, with preschool admissions, and often totaling near the half-million mark) if it weren't. The leaders of our society don't value culture or education that much; they do value tradition, pedigree, and position. Our colleges actually do a pretty good job of educating people who want to learn, but that's a tertiary purpose to the socioeconomic one that justifies tuitions at $50,000 per year instead of, say, $5,000.

The so-called party pathway through college is an all-encompassing lifestyle characterized by virtually nonstop socializing, often on the male-controlled turf of fraternity houses. Substance abuse and sexual assault are common consequences.

College is about protecting the young from an unforgiving society, by handling indiscretions (e.g. substance abuse) with the kid gloves, and protecting their early careers by giving them something close to the best possible work experience they can have at a given age.

A 30-year-old has fits and starts in his career and at least one job that took a painful. A 22-year-old coming out of college has an almost unblemished record. A mediocre GPA can be interpreted as evidence of being social, and while English majors might not place as well as CS majors, literature provides much more interesting conversation than a terminal middle-management position. That is what college achieves. It gets you 4 years into adulthood with a very low risk of having to explain yourself.

Also, we gripe about ageism afflicting the late-career not-that-old, but there's also a strong ageism directed at young adult men. The truth about most 17- to 23-year-old males... is that society doesn't like 'em too much. The social hierarchy is high-status men > high-status women > low-status women > low-status men. (That's part of why gender debates around social justice are so complicated; women get screwed on upside and men get screwed on downside.) Additionally, age and social status are correlated in men, generally upward until old age. Except for wealthy heirs, young men are seen as garbage until they prove themselves. (I'm not saying that this is right; it's clearly not.) College isn't just about making teenage men acceptable for society, but also protecting them during a time in which many people are predisposed to hate them, thinking of them as horny, reckless, socially inept monsters.

Traditionally, college was to protect young men from the horrors that society inflicts upon men of low status, the tricks and lies that throw them by the millions into wars that benefit the rich, and general career sand traps they'd fall into (just on account of being young and not fully formed yet) if they had to fend for themselves... and, also, to protect young women from men below their socioeconomic milieu. (The current clusterfuck surrounding college rape-- it turns out that ivory-tower institutions are completely out of their depth when dealing with a felony that is one of the worst things that one can do to another person-- illustrates, of course, that men of high socioeconomic status can be dangerous, too.) Socioeconomic class is about how long you can delay adulthood and the lack of a safety net and the period of life in which you're held accountable for your choices.

Widespread college also serves society by taking a large number of promising but at-the-time economically useless people off the job market. It protects the students, and it protects the market from their wage depression, and it protects society from a deluge of unemployed people who are (based on socioeconomic status and academic performance) presumed to be dangerous and intelligent enough to be threatening (in a revolutionary sense, i.e. "don't overthrow the government; party on your parent's dime for 4 years").

You're not going to see 30-year-olds in college because, in truth, they're too economically useful (whether they're paid well is another matter) and because they're not deemed to need protection at that age (rather, the young are judged to need protection from them). Also, it doesn't really work. College sells itself as the "get to age 22 without having to explain yourself" option, which is great because most of us would have to explain ourselves if we called our own shots from 18-22... but if you're 30 and just completed your BA, you'll still be asked to explain yourself, defeating much of the purpose.

Unrelenting demand for better-educated workers

Man, I wish. See, if there were "unrelenting demand", then software engineers would have no problem getting themselves sent back to school on full salary for part-time work. You'd see software engineers doing PhDs on their employees' dime as a common phenomenon, rather than a freak rarity. I'm not saying that we should have such a perk and we're certainly not owed or entitled to it, but it's what we'd have if demand were actually so high.

At the upper levels of education, demand is actually pretty low, as illustrated by the academic job market. In fact, the rarity of high-quality work experience is often used as leverage against top talent: they're typically specialized to such a point that few companies are doing what they're good at, and they have to take a cut (in pay or conditions) relative to what they're actually worth in order to protect the specialty.