Obviously learning a couple of words in another language doesn’t really make you bilingual, or being able to say a few phrases. But there’s also clearly some point before full fluency where you can be considered bilingual, but how is it determined (formally or informally)? Is it purely vibes based, you’ll know when you see it kind of thing?
I’m vaguely familiar with the CEFR levels measuring how much of a language you speak, but if there’s a cutoff point for counting as bilingual in there somewhere I don’t know where.
[Caveat lector: I’m not from language acquisition, my main area of knowledge within Linguistics is Historical Linguistics.]
That’s the critical period hypothesis. It’s more complicated than it looks like, and academically divisive; some say that it’s simply the result of people having higher exposure and incentive to learn a language before they’re 12yo, while some claim that it’s due to changes in cerebral structures over time.
And then there’s people like Chomsky who claim that the so-called “window of opportunity” is to learn Language as a human faculty, not to learn a specific language like Mandarin, Spanish, English etc.