(Not sure if this thread belongs here or in the off-topic section. Of course, the mods may move the thread if I put it in the wrong place.)
I think the evidence of that a native speaker of one language can become fully native-like in another language is everywhere. I can't think of any obvious mainstream examples right now -- although I'm sure someone can -- but Mila Kunis (voice of Meg on Family Guy and actress of Jackie on That '70s Show), a Ukrainian, didn't start learning English until she was about 10, and sounds perfectly natural to me. I'd never have guessed that she was a foreign speaker. Kaz Hirai, now president of Sony Computer Entertainment, also sounds perfectly natural, but I don't know when he started learning English and I've heard much less of his speech.
Yudan Taiteki wrote:I'm not sure what you mean by a study of native level speaking ability -- there is loads of research into language acquisition that indicates that once you pass the "critical period", you are no longer capable of learning a language like a native speaker. I imagine any introductory linguistics textbook or a beginning book on language acquisition would cover this.
It is important to distinguish between the critical period for first language acquisition and for foreign language acquisition. These are entirely separate matters, and the idea of a critical period for second language acquisition is much less accepted than it is for first language acquisition.
Yudan Taiteki wrote:But speaking practically, the issue of whether it's possible to gain the language ability of a native speaker is irrelevant, since most people will never come close.
True, for most of us here, it's irrelevant anyway, but I still like to try to get to the bottom of such matters. (Translation: it's fun to debate!)
Anyway, I think anybody who wants to dedicate themselves can do it. Most people, including probably most people who think they want to, actually don't want to do that. It's a crazy amount of work and it's not really worth the effort (unless you'll experience constant discrimination if you don't or something).