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 > 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.