Note I don't ask if this is easy. I ask if this is possible, since it is certailny not easy.
According to this Quora answer at least, it would seem it is not possible - but Quora is Quora.
I am aware of this question: Is it possible for an adult to learn a language without carrying a foreign accent? But my question is noticeably broader, as there are more obstacles to reaching this goal than simply accent.
For example, the aforementioned Quora answer lists subtle word / phrase choices that, even if correct as per the rules of the language, stem from a speaker's habits he has acquired from his native tongue:
there is just a way of phrasing things that is unique to different languages. That’s why it’s easier for non-native speakers to understand other non-native speakers with the same mother language (like two Brazilians understand each other’s English). The grammar could be fine, but you can still tell it’s not native. Since I have experience teaching Brazilians, one example of this that jumps to mind is using “already” instead of the present perfect tense:
I already went to China. / I have gone to China. (or even better, “I have been to China.”)
Is “I already went to China” incorrect? No. Would it be a native speaker’s response to “What countries have you been to?” Probably not.
This is apparently noticeable in cases like the Joseph Condrad's:
In the opinion of some biographers, Conrad's third language, English, remained under the influence of his first two languages—Polish and French. This makes his English seem unusual. Najder writes that:
[H]e was a man of three cultures: Polish, French, and English. Brought up in a Polish family and cultural environment... he learned French as a child, and at the age of less than seventeen went to France, to serve... four years in the French merchant marine. At school he must have learned German, but French remained the language he spoke with greatest fluency (and no foreign accent) until the end of his life. He was well versed in French history and literature, and French novelists were his artistic models. But he wrote all his books in English—the tongue he started to learn at the age of twenty. He was thus an English writer who grew up in other linguistic and cultural environments. His work can be seen as located in the borderland of auto-translation.
Inevitably for a trilingual Polish–French–English-speaker, Conrad's writings occasionally show linguistic spillover: "Franglais" or "Poglish"—the inadvertent use of French or Polish vocabulary, grammar, or syntax in his English writings. In one instance, Najder uses "several slips in vocabulary, typical for Conrad (Gallicisms) and grammar (usually Polonisms)" as part of internal evidence against Conrad's sometime literary collaborator Ford Madox Ford's claim to have written a certain instalment of Conrad's novel Nostromo, for publication in T. P.'s Weekly, on behalf of an ill Conrad.
The above citation comes from Wikipedia's biography of Joseph Conrad
Also I believe my question is different from What qualities separate those few who acquire native speaker fluency, from those who are only fluent? , since, IIUC, this question doesn't assume indistinguishability - while I'm asking if it is fundamentally (im)possible for a non-native speaker to pass as a native speaker (I may be wrong though).
Also while my original intention was to ask about adult learners, we can also include younger learners in comparision to adult learners.