I understand your very well researched post, but a common man such as I would have a different definition of fluency. I have interacted with a friend's child wh

I understand your very well researched post, but a common man such as I would have a different definition of fluency. I have interacted with a friend's child which grew up in a trilingual household and he is speaking my language at the exact level of fluency I would expect a bright 7 year old to have. I didn't try to discuss philosophy with him, because you know he's a child. Of the other two languages he speaks, I do speak one as a A2-B1 level (ILR level 2 I guess) and he sounded to me like a native using constructions that were above my level. So maybe my anecdata doesn't fit the exact scientific consensus, but I am very confident I am not wrong. I would call that fluency especially for the purposes of a HN discussion and not a scientific journal. You might be thinking to apply measurements meant to verify a grown-ups fluency (with all that that implies: vocabulary, idioms, ability to detect nuance, etc.) for children, which is in my opinion a flawed methodology.