At age 14, they're not very neurologically mature, physically mature, or socially mature. This would limit their ability to contribute in the workplace without being a burden on employers. They can't drive and they don't have bank accounts or credit cards. The simple answer is to just lower the age requirement for these things, but I don't want 14-year-olds behind the wheel.
You could also just have another intermediary school, like Germany does with their Realschule, which would be vocational/trade education, but that may still result in overproduction as not everyone would need to go there.
I certainly see the German system as an improvement over what the Anglosphere has, although it's not perfect. The Realschule is essentially what non-banking capitalists want -- worker funded job training. It's exploitative but at least it isn't as bad as worker funded compulsory poetry analysis. My system is similar to the German system plus a more fine-tuned analysis and who needs to go where, minus some of their academic bloat, and minus worker funded job training.
As for the meat of you comment: your conception of teenagers is a socially constructed myth created by the classes I just critiqued. I have some comments on this from last week.
I'll add some more. First, a quote, and then some history: In a dissenting opinion in that case, Justice Antonin Scalia reflected on a 1990 brief filed by the APA in support of adolescents’ right to seek an abortion without parental consent (Hodgson v. Minnesota). In this case, the APA argued that adolescent decision making was virtually indistinguishable from adult decision making by the age of 14 or 15. Scalia pointed out this seeming inconsistency: “The APA claims in this case that scientific evidence shows persons under 18 lack the ability to take moral responsibility for their decisions, the APA has previously taken precisely the opposite position before this very Court. Given the nuances of scientific methodology and conflicting views, courts—which can only consider the limited evidence on the record before them, are ill equipped to determine which view of science is the right one” Another tidbit: that paper I linked? "Giedd" is the one I was talking about in my earlier comments, who made the false claims about the cerebellum.
Now the history. This "New Consensus" that the APA switched to and that you expressed, that emerged around the year 2000 is totally ahistorical. What is historical is 14 year olds either working or going to college. From Youth and History: Tradition and Change in European Age Relations, 1770-present:
In late sixteenth century Ealing, an English village, it appears that boys ordinarily left home between the ages of 8 and 15, while girls were moving out between the ages of 9 and 14. ... This was also the time of life when young men were sent off to schools, apprenticeships, or novitiates in the church. Lawrence Stone has been able to show that sons of the aristocracy entered Oxford at a little over 15 in the seventeenth century, almost a year and a half earlier than did commoner students.
This goes back to antiquity. In fact, things degenerated from antiquity to the middle ages. Youth were more precocious in Ancient Rome. Kleijwegt's Ancient Youth relates that males would don the togo virilis, the garment of adult males, beginning in between the ages of 14-16. Upper class youth would leave home around this time to seek training; as a consequence there are multiple examples of 17-18 year old practicing doctors and lawyers.
When Cicero took upon himself the defense of Caelius, his opponent was the 17 year old Atratinus
He also gives a list of magistrates under the age of 25. There are 25 of them and 3 are 17, 1 is 18, 5 are 20.
The system I propose is very much in line with historical practice; your worries are ahistorical.