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.
"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.