12 year old kids are still developing the brain structures to be able to handle discipline. Meanwhile a large fraction of adults are failing to do what you’re expecting a 12 year old to get right.
When you look around and everyone is suddenly overweight and addicted to their phones humans didn’t suddenly lose willpower, their environment changed.
Is it not because they failed to learn it in there teenage years?
My mother is a teacher and she noticed that kids that regularly do some kind of competitive sports tend to be much more hardworking in school, and it does extend to their university studies as well. Meanwhile "former gifted children" often experience the first year of university as a giant slap on the face, because they never learnt how to study, how to work hard for something, and being smart is often not enough at this level. Many can't even stand up from that hit.
So this is absolutely a huge disservice to not teach children some sort of self-discipline, motivation is never enough, there will always be days when you don't have enough of the latter, and only the former could push you forward then.
Learning just about anything looks very different than handling the full responsibility of doing the thing correctly in your own. ‘How to teach someone to use a cellphone’ is a much better question than ‘is 12 years old enough to be given one.’
Without that you will get the result in your final sentence.
“A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.”
― Robert A. Heinlein
Heinlein is right in principle but its a big ask. can do quite a bit of that list, but I have never butchered a hog or conned a ship or planned an invasion. I am pretty sure I could pitch manure but finding out whether you can die gallantly is likely to be the last thing you find out.
No human has the capability to learn to do all the things necessary to sustain a modern technological lifestyle solo, with the limited time we have on this planet. At best, someone who's wealthy enough not to have to do all the boring, time-consuming parts might be able to learn a decent subset.
Heinlein's purported philosophy fits very well with the all-too-American "rugged individualist" perspective that every person should be completely self-sufficient, but it doesn't actually hold up if you study psychology, sociology, or history.
It is, perhaps, also relevant that this quote is from the book "Time Enough for Love", whose main character, Lazarus Long, has been alive for many centuries.
They really aren't. Brains are not close to being fully developed until the age of 25.
The gift of "adult discipline" is quite a flawed idea. Depending on how far you take it, that's exactly the kind of thing that can create trauma, depression, low self esteem and perhaps worst can affect creativity self expression and just wanting to play.
Play, undiscipline, rebelliousness, is exactly where the Apple Macintosh came from and so many other amazing technologies and ideas came from in the world.
I'd say exactly the opposite, we need to find ways of removing discipline and conformity and extending play and self-expression into adult life for as long as possible as it is the foundation of so much goodness.
That said, if your idea of "Adult discipline" is chock-o-block full of play and self-expression then I'm all ears.
Brains continue developing throughout our lifetimes.
The study that appeared to show them stopping development at 25 did not have any participants older than 25.
It would be convenient to have a specific age we can point to where we can say "now you're fully adult!" based on biological factors, but I'm afraid we'll just have to use our flawed human judgement and draw imperfect lines.
That said, it is fairly well-understood when various of the structures and functions in the brain responsible for certain basic capacities (like discipline) first develop, on average.
Its not one study, its a multitude of studies of a different functions, and the popular conception about “brain development” not being full until the mid 20s is specifically about where multiple studies show the average peak in executive function occurs (with a slow decline after the peak, which obviously wouldn't be seen if it was only based on studies of younger people.)
Other functions peak anywhere from a little earlier, to much later, to, in a few cases, continuing to develop without a discernible age-related peak.
You see my last sentence when you don’t change how our parents were raised. A 12 year old isn’t ready to handle the full responsibility of a smartphone or grocery shopping etc, but that doesn’t mean you can’t introduce aspects of a smartphone.