"it's just like riding a bicycle" So I'll re-learn as an adult much more slowly & with more difficulty than I first learned as a kid, and then I'll plateau at a significantly lower level?

That's been my experience of most things that are subject to that comparison - driving a stick shift, ballroom dancing, and yes, riding a bike. I do form some amount of muscle memory, but my experience of it isn't like what other people describe.

Different people are different.