Earlier quoted context omitted.
> It is arguably the best way to learn the language; after all, it was the way you learned your first language It took me ten years to be as proficient as a ten years old kid in my first language. So I don't think the way we learn our first language is necessary the best way to learn language in general.
You were also pretty dumb at the time, and not trying to learn it very actively. And, I would say I was passably proficient by about 7. At least enough to read relatively simple non-picture books.
1) You have the ability to translate. You already know the concepts and ideas - you don't have to abstract meaning from noises - you already know the meaning, you just have to correlate it
2) You understand grammar concepts. It may be quite different in your target language - I have been learning 日本語 for some time, and grammar is wildly different from English - but there's still a lot of things you only need to learn a few differences of, vs. having to learn the concepts from scratch
With our first language, we have to sit there and learn by what sounds right vs. what sounds wrong. This is so hilariously trial and error and completely inefficient that it makes the entire process far slower than it would be if we had any other way to do it.
How long does it take you to figure out verb conjugation the way you learn a first language? Quite some time. You probably don't get it fully right in all cases until you later enter classes that go over the grammar rules. In Japanese, I was conjugating verbs, including irregular and negative forms, at multiple politeness levels, in about two hours of learning conjugation. And I rarely made mistakes after that. Why? Because I already understand verb conjugation. It's a concept I have a solid grasp on in my native tongue, and only needed to learn a few new rules to graft onto the existing framework.