But you also implied that it's a question of being and adult or not, as if it's a cut-off. It isn't.
If you watch people and you know when they started to learn a new language you can see a pretty strong statistical trend (what I've been exposed to is lots of immigrants who came to the country at various ages): The youngest children learn the language relatively quickly, and there's no accent. If they're around 9 or ten they still learn the language quickly, but the accent may remain for several years. And this gradually "worsen", in a way, as they start learning the language at a later age.
But there are lots of outliers, both ways. There are a few people I know who learned my language to absolute perfection, 100% native in every possible way, in their late twenties.
On the other hand there are also a large majority of people learning my language as mature adults and becoming fluent to the extent that nobody really cares about accent or way of speech - they're not sounding "native", but it's perfectly fine and doesn't matter. People stop noticing, it's just part of who they are.
Though when you get to around sixty it's definitely harder, but still doable.
And it varies between people and it matters a lot what language you're trying to learn.
You may have meant it like you say but in fact you did write
"..Even people who move to another country as adults and spend 100% of their time speaking that country’s language almost never learn to speak it with native-level intuition and a native-sounding accent (though they can reach pretty high levels). Children do, without even trying"
You simply differed between adults and children. If you meant "it gets more and more difficult the farther you move from childhood".. well fine, but you didn't write the equivalent of 'further north than Brazil' so you should expect to get questioned about that.
If you watch people and you know when they started to learn a new language you can see a pretty strong statistical trend (what I've been exposed to is lots of immigrants who came to the country at various ages): The youngest children learn the language relatively quickly, and there's no accent. If they're around 9 or ten they still learn the language quickly, but the accent may remain for several years. And this gradually "worsen", in a way, as they start learning the language at a later age.
But there are lots of outliers, both ways. There are a few people I know who learned my language to absolute perfection, 100% native in every possible way, in their late twenties.
On the other hand there are also a large majority of people learning my language as mature adults and becoming fluent to the extent that nobody really cares about accent or way of speech - they're not sounding "native", but it's perfectly fine and doesn't matter. People stop noticing, it's just part of who they are.
Though when you get to around sixty it's definitely harder, but still doable. And it varies between people and it matters a lot what language you're trying to learn.