Is it possible to become truly Native Level in a secondary language?

Question

I will specify that people born into bilingual families do not count. It must be someone who started learning the language after childhood (where one's susceptibility to language is virtually cheating).

Can someone who learns Japanese, say, with English as their native language be completely (not nearly) natural in both mannerisms, speech, etc., to be perfectly indistinguishable from a native speaker's POV? Meaning, if a Japanese person were to converse with this person on the phone using all colloquialisms and idioms, one would never get the faintest feeling that something was "off" about them?

I'm curious if any major studies have been conducted which examine something like this. It is frequently found that people who have dedicated their life to intensive study and immersion still find themselves occasionally lost in a conversation in that language.

Thank you.

Answer 1

This question is of course a very well-studied one, but with a very important caveat that is revealed in the wording of your question: "Native Level" is a poorly defined concept.

But you do refer to what is called the critical period hypothesis, which breaks down into several maturational constraints, which limit how well language learning proceeds, some of which are linked to age. However, that 2002 study specifically mentions:

...learners who have been identified as indistinguishable from native speakers characteristically exhibit non‐native features that are unperceivable except in detailed and systematic linguistic analyses.

Here we find a (tacit?) admission that it is very much a judgement call, made by social convention of an already defined group of speakers, as to what variation is deemed "acceptable". It has been argued that this may be an idealisation of a standard language, and hence there is no "fundamental difference" between the potential of child learners and adult learners.

This of course is something that chimes in with the experience of those who do have contact with highly multilingual environments: competent "native-like proficiency" is attainable and sometimes even unremarkable, even for those who have passed the various critical periods.