This is a common misconception. It is extremely rare for any child to be fluent in any language by the time they start school.
There are different levels of fluency that language training professionals recognize, mostly standardized on the ILR scale [0].
There are 5 levels on this scale. What people commonly consider "fluent" is level 3. There are also midpoints recognized by a "+", e.g. most people who study language seriously but don't have any special aptitude for language training get to a 2+. It's an open question of serious debate and active research of whether it's possible to train anyone past 2+ to 3. Some rare individuals seem to have an innate aptitude for language acquisition and they seem to breeze through their training to 3 and then continue on from there. Everyone else seems to plateau at 2+.
3 is just the level of "can easily be understood as a foreigner by a native speaker". It's not native level of mastery. "So fluent you can fool a native speaker" is 5.
All of that is to say that children are most definitely not fluent by the time they reach school age. Most children starting school will be at a 0+ level. Particularly intelligent children might be at a 1. But they are definitely nowhere near 3.
And indeed, by the time most people graduate highschool, I would struggle to admit that they are much beyond 3. Hell, I'd even say a large portion of people graduating college don't have mastery of their native language.
So no, I don't think children have a particularly large advantage in learning language. Despite significant, continue training in language for the first 18 years of their lives, most aren't able to get very far beyond what any studious adult can learn in 2 years.
That is not a definition of "fluency" I'm familiar with. To me what you're describing is about "proficiency", and the latter is the term used by e.g. CEFR, I quote: "The CEFR organises language proficiency in six levels, A1 to C2".
"Fluency" is when you can easily speak without having to construct the sentence in your mind, not even for a moment. It just comes out. It _flows_, which is what the term originally meant. Being fluent does not imply that you can talk about anything and everything from philosophy to Bach, or whatever.
To hear "It is extremely rare for any child to be fluent in any language by the time they start school".. that makes no sense. Of course every single child is fluent in _at least_ their native language when they reach school! And long before.
Some are fluent in more than one language, but it's much rarer. Though I do know a family where all three children were fluent in three languages before they started school. Special circumstances though, and thoroughly anecdotal.
I was fluent in 3 languages before school, as is my little brother. Parents from 2 different countries, and moved to America when I was a young child and my brother was born in the USA. It helps if the parents can communicate to each other in their native tongues (like my parents). Of all the 2+ language families I knew growing up, the children generally only learned their parents' native language if both parents either had the same native language, or if both parents were able to speak both native languages. Also thoroughly anecdotal.
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.
There are different levels of fluency that language training professionals recognize, mostly standardized on the ILR scale [0].
There are 5 levels on this scale. What people commonly consider "fluent" is level 3. There are also midpoints recognized by a "+", e.g. most people who study language seriously but don't have any special aptitude for language training get to a 2+. It's an open question of serious debate and active research of whether it's possible to train anyone past 2+ to 3. Some rare individuals seem to have an innate aptitude for language acquisition and they seem to breeze through their training to 3 and then continue on from there. Everyone else seems to plateau at 2+.
3 is just the level of "can easily be understood as a foreigner by a native speaker". It's not native level of mastery. "So fluent you can fool a native speaker" is 5.
All of that is to say that children are most definitely not fluent by the time they reach school age. Most children starting school will be at a 0+ level. Particularly intelligent children might be at a 1. But they are definitely nowhere near 3.
And indeed, by the time most people graduate highschool, I would struggle to admit that they are much beyond 3. Hell, I'd even say a large portion of people graduating college don't have mastery of their native language.
So no, I don't think children have a particularly large advantage in learning language. Despite significant, continue training in language for the first 18 years of their lives, most aren't able to get very far beyond what any studious adult can learn in 2 years.
[0] https://en.m.wikipedia.org/wiki/ILR_scale#ILR_scale
Edits: damn glass keyboards making my own fluency look suspect