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).
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.