This is actually a somewhat controversial claim, academically. There's pretty strong evidence for a "critical period" for acquiring a native-sounding accent, and some evidence that certain adult learners still occasionally mess up minor grammatical details after decades of immersion. But other studies suggest that language-learning ability trails off gradually with age in much the same way that, say, physical ability does. A 35-year-old can still get in pretty good shape if they work at it.

I know plenty of people whose language-learning strategy is basically, "Put myself in situations where I need to use the language and I can puzzle things out. If I get curious, maybe look something up once in a while." It's not quite like childhood acquisition, but neither is it a completely different process.

I strongly disagree, and the education system approach is provably a massive failure in this regard. That's all I'll say for now.

"Provably" how? Many people learn through a class and go on to speak a second language, so what's the proof?

Sure, eventually you just have to use it. But as an adult learner, you can get going a lot faster by using your knowledge of other languages, understanding of rules, and so on, before jumping in, and it's not always practical for people to just take off and spend months or years in a foreign country in the hopes they'll learn a language while they're there.

The grandparent's point is correct, although he didn't say anything about formalized education. You inserted that yourself.

You can't just "disagree". This is a matter which science (linguistics specifically) has lots of research on.

If you're just saying that they're not perfectly identical, then that's obvious, you've said nothing. What I'm replying to is your implication that they're vastly different. Science shows that they're not as vastly different as the education system assumes.