What is a “Native Speaker,” Anyway?
I have a friend, Carlos, who is a professional programmer, and so he is quite fluent in English (like most programmers, I reckon). He’s a Brazilian, but he’s worked in an English-speaking context for years, and his English is flawless.
So here’s the interesting bit: he pointed out to me today that he actually sometimes finds himself struggling with bits and pieces of Portuguese grammar. (Fellow brazucophiles, the example he mentioned was that bastante is sometimes pluralized… did not know that.)
This got me thinking (again) about just what it means to say that you are a “native speaker” of a language. In the translation industry, it’s pretty much a standard that you should only translate into your “native” language.
Whatever that means. Who am I to disagree with someone who’s lived in a country for 20 years and claims that they are a “native” speaker of their second language?
I guess what it comes down to is that the term “native” is kind of icky… for one thing, it sounds geographical. I wish there were a better term. “Fluent” doesn’t capture what I’m after. So far I’ve settled on “Native or native-like proficiency” and similarly unwieldy phrases.
What does the “native” in “native speaker” and “native language” mean to you?

For me, it all depends on the age in which a language is acquired. In the development of the brain, at about 7 (?) years, the specific capacity to learn a language as a “native” language is lost.
I’m also in this situation. Until age 27 I spoke only my native language which is French. I then moved to the US and immersed myself in the English language to the point of never speaking French at all for years.
I have done a whole lot of public speaking, teaching philosophy and within a few years I found that I was not able to express myself in French as well as English. Obviously I understand French perfectly but I get stuck on grammatical points fine points and vocabulary wise, words pop into my head in English. Needless to say it’s embarrassing when you meet your friends, family, etc.
Having been around many folks in a similar situation as mine I came to the conclusion that those who keep speaking their native language everyday while living abroad do not develop an accent or lack of ability in expressing themselves whereas those of us who never speak their native language do lose, albeit temporarily, the command of their mother tongue.
I would define a native speaker as someone who is speaking in their mother tongue.
Must have learned the language during the critical period, i.e., from infancy to puberty.
In my case, there is no question what my native language is. Even though English was the second language I learned (starting at around age 3) it is the only language I can think in, speak without an accent, speak and write without non-native errors, and read with ease instead of with effort.
As for writing—and translating into—a language, I think the only criterion that can count is a sort of Turing test: if a text can’t be identified as written by a non-native speaker, then the writer can count themselves as native-equivalent.
The cut-off age for acquiring a native language, though, surely differs from person to person, and may also be influenced by continued exposure or lack of it to the previous language. My father and mother had their first exposure to English at ages 16 and 8 respectively, and by the time I came around, they spoke English like native speakers, that is, without accent, and without non-native errors (actually without errors at all).