Similar theory I have is that it's acceptable for most children to learn their language full-time; whereas for adults, usually it's a side thing on top of their full-time work.

If given the opportunity to immerse in language learning full-time for 3 years (time it takes for most children to speak somewhat coherently) and with an open mind, adults can react fluency, I think. Maybe even faster than children, since adults have more context to bind and commit new information to long-term memory.

As a kid, memorizing vocabulary words was a brute force task because I had no relevant context (Latin/Greek, history, etc), but now I can pick up words, even foreign words pretty quickly because I have "hooks" in my mind to which I can attach them.