My background is that I have several years experience working as a Linguist for the govt. I speak 5 languages fluently plus another 5, I barely speak and/or understand. The 5 that I speak fluently, I learned by the time I was 12. The other 5, I picked up here and there and put no hardcore effort to learn or improve and learning remains so very stagnant. I believe that if my only job was to play with locals and learn languages, then I would be able to learn new languages and gain fluency in a decent timeframe even now.

Regarding children vs. adult learning debate, I would say that learning how to speak a new language is much easier for children. However, after a certain point it is schooling (college/DLI/home study) that makes a person refined in a language, that is where the adult learning part comes in place. I picked up Italian in Italy during my high school years and when I was in college in the US, my Italian professors would always tell me not to speak/write in Roman slang and deduct points for that. That is where I think the adult learning is crucial. If a child lives abroad for 18 years, picks up a language just by speaking with people/watching television etc., that person will most probably speak fluently. However, unless they are reading books and writing a little, they will not be able to pass a 4+/5 level reading/writing proficiency test because those tests are hard and are at college level or at least high school honors level. So the testers will most probably not pass a person at a native level if one starts speaking a more street version of the language picked in back alleys or slang picked up in high school.

To Mr. Dayuhan's question about multilingual individuals learning a brand new language as adults, I would say, it is not as easy as learning while younger, one reason is definitely not having time to play with locals :-)

If all other things being equal, a hard-working, eager to learn monolingual and a lazy multilingual who doesn't put any effort are put in a class to learn a language with no prior exposure, I would think that the monolingual would fare better just by the sheer effort to learn?

I salute all of you who are leaning new languages and putting an effort, most specially if you didn't grow up in that linguistic environment.