I've never understood this belief. There are many things I struggled with as a child that are very easy for me to understand now. For instance, calculus. I struggled barely passing my classes originally. A few years ago I decided that I really needed to know calculus so I bought a book and worked through all the problems. Not only was it not difficult but the whole thing just made logical sense; it was all straightforward.

I can believe there are some things that would take me longer as an adult to learn than if I were a child (a new language for instance) but origami folding wouldn't be one of them.