[wccrawford]

As the other comment says, it's all about the immersion. If you're locked somewhere with no way to communicate except to learn a new language, you'll learn it pretty fast. There might be an upper age limit on learning it fluently, but I wouldn't bet on it.

If anything, adults have an advantage that they have settled down a bit and can study effectively on their own, instead of just passive and forced learning.

Of course, they still have all the temptation to just screw around, too. And if you're not actually locked to that new language, the old language is incredibly tempting.