Imo this is so overrated, the biggest difference is that children have 100% exposure and almost all language they come across is tailored exactly to the right level, just for them.

this is massively overrated. Given the same amount of exposure, adults win every single time. Just look at the other people in this thread decrying the idea that it could possibly take 5 years to learn a language. How fluent would you say a 5 year old is? Consider their mastery of legal terms, cooking, vocabulary, geography, political terms, things from all aspects of life? Their grasp of grammar and spelling?

You might suggest "oh but surely a child knows far more about animals and plants and countries and the like, child things" but I find almost all adults pick these up extremely easily, at least the same ones a child might be expected to know. (lion, tiger, blue whale, dinosaur: Yes. Osprey, Bullfinch, Plaice, Tapir perhaps not.)

However, adults must put a lot of very draining, effortful study consistently to have a chance. I'd put learning a language to fluency on the same level of difficulty as getting a degree. Something one really must do if living in another country, but not something to be trivialised, or sneered at someone for not having found time to complete yet.

I think the biggest difference is that children don't have mental barriers about those things. They just explore and learn, without any thoughts wasted about the usefulness, pressure or excuses not to learn.

Just as importantly, they have little to no alternative. And then they are given every opportunity to learn—at school, by parents, and so on.