I wish I would have learned an instrument when I was a kid but unfortunately I did not. I've tried several times as an adult but I always give up after a short period of time. I can't say if it's true or not that you don't forget something learned as a kid but I suspect that it is nuanced like you say. Neurons (brain cells) form networks in the brain and the more they are used over time, the more ingrained they become. They become larger and faster. On the other hand, networks that are not used get weakened over time. But the networks also retain memory as well, and when a once strong network is used again after a long pause, it will re-form quicker than one that is starting from scratch. So it's like you said really, it all depends on how well something was known and practiced in the past; how strong are the neural networks in the brain.
It is also nice to try to learn as an adult, but there are more limitations. I see it from my adult students. Usually, there is not a lot of time for studying, as life, work, family ask for their priorities. And that is pretty normal. As a child, your work is to study, although it is not always like that either hahaha 😂
You have made a super nice summary and added a bit more sense. Yes, yes, absolutely agree with this, networks that are not used become weaker, and when once it was strong and with a good base and memory, it can be reformed quickly! NOt so quickly everything, as I mentioned in some other comments, when something indeed very hard, technically hard, it is not easy to regain the same ability.
I somehow compare it now with athletes and achievements. The difference is what they can do when young and when older. I am afraid, even if I would practice again 4, 5, or 6 hours a day (as I used to do it 20 years ago) I think I would not be able to play again with the same ease the extremely hard compositions.
But, I should give it a try to know it 😂 Still not giving it too much hope as those hours for piano don't exist in the daily schedule 😆