I think aiming for “native like” fluency is not a good idea. We are highly unlikely to ever reach it even if we move to the country. But that doesn’t mean we cannot learn fast
And why should we aim to speak like teenagers? We can speak like adults, we can speak very well. the CEFR scale doesn’t use the term native-like and someone at a high level (C1 or C2) can function in the language more or less like a native of similar age and education.
Whether we can learn fast or not doesn’t have anything to do with this. Someone aiming just for the intermediate level (around B1) can still get there very fast.
Of course most people quit, just like many people quit sports lessons, or music lessons, and just go for another activity. And normal highschool classes simply don’t lead to the C1 or C2 level in vast majority of cases, no wonder you didn’t get there either, if you didn’t serious study outside the class too and if you quited after school, instead of building on the already acquired knowledge.
I think a part of the problem is the definition of what does “fast” mean. In case of Japanese, not being “fluent” after 15 months doesn’t mean you are slow, you are probably well within the norm. 15 months are a short time to get to C1/C2 even in “easy” langauges. The learns vary a lot between the “very slow rate” and “extremely fast rate given by intelligence, ideal external conditions, and tons of hard work and invested time”.
I have reached C1/C2 levels in two foreign languages so far. In one more my passive skills are there too, and I am trying to catch up with the rest. I have passed exams proving my level in those two languages. And I can function in all the areas like a native person of my age and education in those languages, despite having an accent (not hindering comprehension at all), making occassional mistakes, and the fact I’d need to learn a lot of facts about living in the countries, should I live there.