Because I married a Frenchman, I live in France and my children were all schooled in the French system.
One specificity of this system is the amount homework children are expected to do from the age of 6 onwards (even though there are laws against it !)

Originally British (well actually English 😉 I was totally amazed when my 6 year-old came home with the task “learn off by heart: je suis, tu es, il est, nous sommes, vous êtes, ils sont“

“What?”, I said to myself, “a French child in France learning French in the way I was taught French as a foreign language”  I just didn’t get it at first.
My friends and neighbours were all amused by my reaction. “But we all learnt it like that at first” they told me….Stranger and stranger.
The child knew how to use the language, and yet was now being asked to do something which, to me at that time, was French as a foreign language.
But  I didn’t know what was coming next – by the age of 11 she was expected to recognise a “proposition subordonnée circonstancielle de lieu / de temps” which, I learn from a French University’s English grammar handout  is in English, an “adverbial clause”
Now a question for NESTs reading this, Do you know what are the subordinators of abverbial clauses of place / or time ?
Well actually, of course you do, but you just didn’t know that that’s what they’re called!  Adverbial clauses and the like are, for a native English speaker, in-depth meta-language.
SOOOOO my point here is NOT that, by learning this “meta language”, by learning to describe this system of grammar, a 12 year old French child can ask themselves (or have explained to them) the difference between “où” and “ou” (“where” and “or” in English).
No, my point here is that having had grammar lessons from the age of 6 to the age of 15 years old (the grammar lessons stop when the student enters the “lycée”, i.e. K10-K12 in US speak, and (almost) Sixth Form College in Brit-speak.) gives the speakers of this language a tremendous advantage when learning a foreign language, since they already have a meta-language with which to speak about it’s structure.
From this, I hazarded a guess that, the more grammatically complicated a person’s L1, the easier it should be to learn a foreign language. And this kind of seems to hold, when one looks at native speakers of German, Georgian (reputedly the most difficult langauge to learn), and other “complicated” languages which add declinations to congugations etc. These people are so soaked in a means of talking about how the language is structured, that they have a head start over native English speakers when learning a second language.