Finnish or not, it is orders of magnitude harder to learn any language as an adult.
a) convincing yourself its worth the effort: almost every time an adult runs into a confusing element of a new language, they find themselves calculating how many people in the world speak this language, probability they don't speak english and likelihood of running into this person and circumstance, and it's easy to justify giving up and moving on
b) avoiding forcing it into the framework of your first language: if you have one distinctly favored language already, it's very hard not to try shove the new language you are learning into the former's mold, and this can be counterproductive in learning most languages that don't share an ancestor with your favored one.
a) is greatly mitigated by forcing yourself to be in said context by living in a place prioritizing that language. b) is greatly mitigated by already being bilingual+ with languages from distinct origins (eg: mandarin chinese and english) before learning a new one, so you can place the new language on a spectrum with the ones you already know instead of confined by the rules of just one.
An adult studying a language is spending like maybe 1% of the time studying that a child learning a language spends.
The first comment seems to be arguing that the difference is "literally just a function of time and exposure opportunity and nothing else" (explicitly ruling out a significant effect related to a difference in brain plasticity related to age), while the comment you're answering to argues that brain plasticity, even under the same conditions of time and exposure opportunity, makes a significant difference.
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.
No I didn't. I never said anywhere that it's a sharp cutoff rather than a sliding scale.
If I say Canada is further north than Brazil, it doesn't mean that latitude isn't a continuous spectrum.
"..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.
So, 'adult' isn't really right in my opinion / experience - I know many people who were adults and could absorb another language relatively easily.
What seems to happen is that it does change for the worse as the decades go by. But it's not as simple as adult vs child. And it's not by orders of magnitude in any case, it's more linear than that.
I spoke four languages by elementary school, including learning a new one (portuguese) in fifth grade. It was all seamless, zero effort, perfect local accent. So I have plenty of language background.
I've spent almost 20 years now as an adult trying to learn french and find it impossible. Still speak it at a 2-year old level, at best. I have constant daily exposure to french, it's just an impossible language. Or that learning a new language as an adult it impossible.