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@MMM. I didn't say anything about persons who lived abroad and never learned the language of their hosts, but to rush conclusions by only these kind of examples is quite unfair and also, you compared him with a child under 5, which made your example even more unfair.
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Theory is one thing reality is another. No children under the age of 6-7 would be able to learn any foreign language without parental or 'environmental' help, in the cases when it seemed that one learned 2L without help won't count, the reason was: the child watched TV all day long, namely cartoon channels in a foreign language, so it was like having a 10 hour course/day or living abroad.
And even in a teenager's case to start with better ops, he/she needs really good education-system, otherwise he'll end up with false or inefficient knowledge. Also if you consider the fact teenagers have real hard time to concentrate on anything (because of certain physical/hormonal and intellectual issues), and that: half a year in a university basically contains four-five years of high-school knowledge (from my experience); I'd say the theory is OK, but in reality a 17years old student have only just begun his studies. That's why I said he'll be able to concentrate more and more on his goals, but the opportunity to reach the level of a native speaker is long lost, so why would I mention it?
On the other hand a grown up, whose determination is solid (because he has to work, he has limited time to spend, and has a real idea why it is necessary to learn etc.) will find the way to obtain the language better.

About neurons and psychology, those are very context sensitive stuffs. The wild-child theory is extreme and still, as many cases as many results, one of them died another could adopt the requirements of the society. It's true there are examples which say that is going to get harder to learn anything or even impossible as a person's aging. But to say it generally shuts down the opportunity to learn is just not true, not even in the case of languages, it all depends on the individual- on his knowledge, his physical and mental state and the environment he's in, and non the less on his determination!