If memory serves, there are different levels native multilingualism. If you learned multiple languages in parallel, they end up sharing space on the neural structure. I would believe this is "true" multilingualism.
If you learned one separately from another, after the first was stable already, the new language will occupy an adjacent region (somehow I think this is because the region is still growing).
All languages will be functionally native (assuming early enough acquisition and abundant practice for all), but in the latter case, you would still expect a slightly weaker signal for the later languages. But in both cases, I would just count all native-level languages as first languages, taking "minimal accent" as the litmus test (some people slip through, and in my experience those people had amazing ears and were gifted with languages in general).
This is just anecdote, but from all my friends who have began learning English at a different age, their accents seem to follow a general pattern of later acquisition -> stronger accent. While this seems obvious, for those who learned English around 12-14, you get an interesting effect of native-level fluency, but just at certain places, you can tell it isn't their first language. And usually, at least for my friends, you can tell about what age they started learning.
If you learned one separately from another, after the first was stable already, the new language will occupy an adjacent region (somehow I think this is because the region is still growing).
All languages will be functionally native (assuming early enough acquisition and abundant practice for all), but in the latter case, you would still expect a slightly weaker signal for the later languages. But in both cases, I would just count all native-level languages as first languages, taking "minimal accent" as the litmus test (some people slip through, and in my experience those people had amazing ears and were gifted with languages in general).
This is just anecdote, but from all my friends who have began learning English at a different age, their accents seem to follow a general pattern of later acquisition -> stronger accent. While this seems obvious, for those who learned English around 12-14, you get an interesting effect of native-level fluency, but just at certain places, you can tell it isn't their first language. And usually, at least for my friends, you can tell about what age they started learning.