There are different levels of investment and different level of expectations, and I say this as someone that speaks 3 languages (Romanian, English, French) and understands a few others at a basic level (German, Italian, Spanish).

Children have a very limited vocabulary and tend to make a lot of mistakes (grammar, word usage and pronunciation mistakes) and those mistakes are easily overlooked, especially by adults, since we tend to try to encourage them. Not the case for adult learners.

The second difficulty level is <<cultured>> native speaker. This is massively more difficult since you need to advance from maybe a basic 5-10-15k words to probably 20-40-60k words.

The investment level for C2 (proficiency according to https://en.wikipedia.org/wiki/Common_European_Framework_of_R...) is in the thousands of hours per language, including a ton of deliberate study.

To give you an example of C2, for Romanian you need to be at least a well educated high school student and really understand finer details of Romanian grammar. A native Romanian speaker who hasn't finished at least high school at a good high school is probably a bad C2, despite being a native speaker (or more likely a decent C1).

Now, you obviously don't need to be at the highest level for every language you know, but the gap in effort between levels is huge and people underestimate that. Most people that speak multiple languages are probably somewhere around B1-B2 at most of them. Even purely bilingual kids that learned both languages from birth have idiosyncrasies monolingual native speakers notice, for example.

The CEFR is a tool to evaluate proficiency, but it's misleading if you look at it as a "ladder" with "native" at the top". Most adults aren't C2 in their own native language, despite "investing" sometimes hundreds of thousands of hours "learning" it.

This video [https://www.youtube.com/watch?v=uqj5qPF_puU] by Olly Richards sums up my feelings on the definition of fluency. Personally, I believe even as early as B2 you can call yourself a fluent speaker, because it's roughly the level where you stop struggling to express your thoughts or to understand other people's. But it also depends how you have learned. B2 can look very different depending on whether it was leared in mostly academic vs literary vs spoken contexts.

> Even purely bilingual kids that learned both languages from birth have idiosyncrasies monolingual native speakers notice, for example.

Those tend to be cultural. My native language is French, but I cannot browse /r/france without wondering wtf everyone's talking about. I'm extremely disconnected from French culture as I moved out when I was 14, so I never experienced adulthood in French and I had to learn things like what "Pôle emploi" is or some such. I'm still C2 in French,

I also do make mistakes in English, but I catch plenty of native English speakers make egregious mistakes or not know fairly basic things about their own language. Another commenter mentioned the their/they're your/you're distinction is one that natives tend to mess up more often than non-natives; anecdotally, that seems correct to me.

> Those tend to be cultural.

Language is culture. A large part of language learning is learning culture, expressions, idioms.

If I say"white as cheese" to a Frenchman he won't understand what I mean because the average cheese in France is not white, unlike in Romania.