At present, in the West, people usually go on to higher education at about age 18-19; and there they study general (non-vocational) stuff.
This is probably a mistake, from a educational perspective.
General education should be over and done during the teens; and by 18-19 people should be specializing.
In the middle ages, when universities were invented, most undergraduates were in their mid-teens, and this situation persisted in Scotland until the mid-19th century; and a similar situation (modelled on the Scottish system) prevailed in the USA, Canada and Australasia.
Undergraduates studied a standard general curriculum based (broadly) on the Trivium (Grammar - i.e. Latin plus/ minus Greek; Logic and Rhetoric) and the mathematics-based Quadrivium (arithmetic, geometry, harmony and astronomy).
When this was out of the way, the student moved on to higher professional education: theology, law or medicine.
In England, after the middle ages, this picture became confused by the fact that grammar schools pretty much took-over general education, so that by the time the scholars (a small minority of late-teenagers) went on to Oxford and Cambridge, they had 'done' the trivium/ quadrivium subjects and were ready to commence what was de facto post-graduate, advanced studies - typically in Classics or Mathematics.
The 'tutorial system' for scholars at Oxford and Cambridge therefore developed from the late 19th century into something rather like a post-graduate supervision or mini-seminar. (In the post-medieval, pre-reform years the individual or small group tutorial/ supervision was actually given by private coaches or 'crammers' and and was not a critical examination of sources, a far ranging discussion or anything like that - coaching was narrowly devoted to getting the highest possible marks in examinations.)
Meanwhile, in Scotland and the US and most of the Anglosphere, schools did not take their pupils to such a high level, and colleges/ universities retained the medieval 'school' function, with mid-teen students.
It was therefore quite normal, indeed even 40 years ago, for first degree graduates from Scotland, Canada, Australia, New Zealand (as well as the US) to do 'another' undergraduate degree in Oxbridge - since relative to them Oxbridge teaching was pitched at (or near) a post-graduate level.
So the real difference, which made possible the unusual system in England, was the highly academic grammar schools - the English grammar schools (and later on the scholarship classes in the reformed Public Schools) - which by virtue of high selectivity and beginning specialization at about age 14, probably took students up to a higher level than schools in any other Western country. In other words, they completed the students' general education, and they made them ready for specialization.
Even now, England is very unusual in that medicine, dentistry, veterinary medicine and law (as well as theology) are all mostly undergraduate degrees - typically started at age 18-19 and finished age 21-24. In most places they are post-graduate subjects - begun at about age 22 or even later.
At any rate it is clear (from centuries of experience) that the most academic kids attending selective and rigorous schools can normally complete their general education by about 18-19 and commence specialized 'graduate school' type study at that age.
This leads to specialists who rapidly become well-equipped to embark on high-level and independent work at an early enough age that they have a fighting chance of achieving something in the field!- or at least of earning a living by exercise of their specialized craft.
It seems likely that this educationally-efficient system was an important factor in the high rate of top level academic attainment per capita which the English educational system used-to attain.
The advantages of early university are educational, and relate to the greater aptitude of the most academic teenagers for learning and studying, and their superior and longer-lasting memory.
This would require reinventing the highly selective grammar school - and recognizing that such schools can move their pupils through the curriculum *much* faster than at present, by specialization. And of course the university curriculum would need again to be linked to the school curriculum - which is achieved by giving elite universities control of the school examination process.
The sooner that the most academic kids tackle advanced subjects, the better. The disadvantages are merely 'social' in the sense that nowadays college social life has evolved to be suitable only for 'adults' (i.e. those legally above the age of consent) - but maybe shattering that association would be an advantage too?
This is, of course, not intended as a serious proposal for reform (indeed, I tend to the idea that secular universities are near the root of our present societal troubles, and that tinkering with the current structure cannot achieve anything worthwhile).
I intend simply to show how far away from a rational and coherent educational system we have drifted.
The current situation in relation to formal education is one where we apparently accept the waste of huge proportions of young lives and vast economic inefficiency.
To this I am here adding a reduction in educational effectiveness. As a general rule, if kids are not educated as highly as possible in their teens, then they never will achieve their best potential. We cannot afford to waste these years.
(But the biggest problem is that of purpose - the reason why mass education cannot now be reformed is that (beyond basic 'literacy') we do not know what formal education is for.)