[hakeberio]

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.

[porknubbins]

I think your theory has been pretty well proven with the huge online polyglot community who learn foreign languages regularly as a hobby, as well as other adult language learners like diplomats, random expats etc. You can even look at institutions like the US govt defense language research institute who give specific times to learn specific languages for a native adult English speaker.