But Progressing through to expert or world-class? History verifies the conventional wisdom that ages ~12-30 show the greatest gains.
And when you look over to things like language and music, the difference in learning ability is vast. Kids can become trilingual effortlessly, while adults almost never get close.
Laszlo Polgar's experiment is tangential but an interesting aside here at least on the notion of 'conventional wisdom'. Polgar wanted to perform an experiment on the nature of mastery of something (mathematics/music/chess/etc) being a matter of work rather than some natural talent as was, and remains the view of 'conventional wisdom.' After some consideration he decided to use chess, something he had no particular talent in, as his test bed -- and announced beforehand that he would create a master. He found a wife (also of no meaningful chess talent) willing to participate and they had a child which was their experiment subject. In fact the experiment was made doubling interesting by the fact that he had a girl. Then another. Then another. "Conventional wisdom verifies" that girls tend to do worse at chess than boys. Those children were Susan[1], Sofia[2], and Judit [3]. He created 2 grandmasters, and one international master. Judit becoming the 8th highest rated player in the world!
Language is another interesting example. I enjoy traveling and know many people that became fluent in languages as adults. And the vast majority tend to share one thing in common - they were forcefully immersed in the language. Cutting out the abstracts there - they tend to have wives or girlfriends that speak little to no english. There's no chicken or egg there, they invariably learned the language after moving in with the person. I expect you'll find the children that are genuinely trilingual (and not I took 4 years of spanish in school and can kind of speak it passably type 'fluent') will be made up heavily of children where multiple languages were spoken in their household - the same sort of 'forcible immersion.'
[1] - https://en.wikipedia.org/wiki/Susan_Polgar
The difference in circumstances is likely greater than the difference in brain plasticity past a few years old.
This doesn't mean learning ceases past the point (clearly not). But it does mean that changing synaptic weights and forming new synapses comprise the bulk of learning rather than new neuronal projections and creating new neurons of which the later two become insignificant with small exceptions (such as the generation of new neurons in a small region called the SVZ and also in the hippocampus, a region important for memory though I don't know if the role of these new neurons is known in any great detail).
Adult learning necessarily uses restricted mechanisms compared with children and adolescents. It's still a general learning machine though, though rate of learning might suffer (I'm not familiar with that part of the field).
However I suspect that everything else being equal children do have a higher skill ceiling because there are very few late achievers in chess.
An experiment would be needed, take a group of 10 year olds, 20 year olds and 30 year olds and immerse them in intensive 5 year course of study.
I would wager that the 10 year olds as a group would have progressed further.
Why do I say so? Because, it is incredibly hard as an adult to obtain the so called unconscious mastery in any field. (Ie what Gladwell alluded to in 'Blink'). It is the pattern recognition that's been alluded by many.
With kids this can happen, 'the jump' so to speak. With adults I've not seen it happen in almost 40 years of playing chess at a decent level(FM).
I remember it somehow clicking for me at 14 for chess. It is the level when you do not have to think and you can make a good move. It was the level when I realized I could keep up with IMs.
Of course you need both pattern recognition(GM-RAM) and the ability to calculate. Ability to calculate can be trained, but it is also part of talent/ability.
Among my kids and nephews only one has that ability.
Are you going to tell me that among your adult students all have the raw ability/potential to make master?
In my opinion, I think anybody (of at least 'normal' mental capacity) has the potential to make master. Will any of my adult students do so? Absolutely not. They want to improve at chess, but want to do so in a way akin to taking a pill. Like you obviously know there is an immense amount of work to improvement and chess improvement is extremely weird. For instance extensive tactics study are key for improvement, but it's not like you'll see it instantly reflected in your rating - and after some time of diligent work and effort, it's entirely possible that somebody's rating might have even decreased. Something I experienced, as well as most other stronger players I know, is the 'stair step.' You don't improve steadily and linearly, there's these weird huge jumps where it seems like somehow you wakeup one day and are just xxx points stronger.
People understandably expect proportional rewards to proportional work, but that's just not how chess improvement works. I also think this is how so many adults get trapped in opening study - there the rewards can be immediately visible, even if it's not really all that helpful to your overall improvement. Kids don't really think about these sort of things and often just enjoy learning about the game and solving the problems. But adults are generally doing it as a means to an end and have much more difficulty dealing with the nature of improving at the game. And it can also directly lead to self doubt in them as they use this as an indicator that they've plateaued, lack talent, or whatever else. And lacking confidence in chess is a great way to kill your results.
But the all else being equal part is the catch. Adults often don't put as much effort into the work as children and a big problem is preconceptions that people have difficulty letting go of.
I also consider myself some slight degree of proof of the hypothesis. I didn't even learn how the pieces moved until I was 17, which is ancient in the chess world, and didn't take it seriously until I was in my 20s. And now I have enough grandmaster scalps to create a.. err.. something that uses lots of scalps!