This is an efficient summary of the social attitude I'm criticizing. People believe that children and teenagers are useless. They believe this because the structure of modern education requires young people to be useless. Every adult in their lives repeats variants of "you cannot and should not be trusted to do real work that other people need", and so young people grow up believing that.
If instead of resenting their presence and demanding that they be warehoused away in the name of efficiency, adults in contemporary society ought to look forward to training young people to be useful. If you take this attitude, and regard every young person as a future ally in your endeavors to maintain civilization to a high standard, it becomes clear that what's being done in most schools is inefficient, poorly-managed, and counterproductive.
Approximately no company wants to deal with an unskilled person. It takes a pro weeks to even start doing something useful in most organizations, let alone to be an useful addition to the team. If teenagers could be expected to produce useful code, then we'd have companies hiring new, unexperienced developers, but pretty much nobody wants to.
Now of course there are some really clever 12 year olds out there that can actually do cool things. But they're a very small proportion, and if you only deal with them then it's no longer education, but a mentorship for a select few.
"Nobody actually wants stuff like that for a serious purpose" is wrong and dismissive. Sometimes they actually do; sometimes they have a real but non-critical need of debatable seriousness, and software from a kid can fill it; sometimes the other person is a fellow 9 year old, what they want is an overly enthusiastic Harry Potter fanfiction where he meets Lightning McQueen, and both the author and reader are happy.
I don't think the suggestion was that students be employed or economically productive, just that they would be producing work to be proud of a lot more often in a better system.
So at around 13 I'd say that I was about the only one of my 30-ish classmates that was actually producing something that resembled working code. I coded a whole functional (but simple) sprite editor in QBASIC that could write PCX files.
But honestly, that code was complete crap. I was barely figuring out what functions were good for, because most of that was being self-taught by reading the manual and trial and error. And compared to my classmates that was top 1% stuff.
Nobody actually wants stuff like that for any serious purpose. No company wants that code, or being saddled with a few dozen teenagers, some of which are completely useless. And that goes for all fields. Who out there needs a human to do trigonometry these days? What about history, or literature? What does production there look like? Do we grade them based on likes and reviews on fanfiction.net?