In term of speaking and conversation may be.
But I doubt we would be able to write hiragana, katakana, or kanji just by watching anime.
Well, no. But reading and writing and learned traits. That's why you can still get illiterate adults. Illiterate adults can still speak and understand the language, they were just never taught to read and write their own language. But, despite their age, illiterate adults can still learn to read and write.
Our adult brains may not be as spongy with information as children, but if we were dumped in a foreign land, given time we could learn the language by ear and experience. Taking lessons would still help, but especially for reading and writing.
I doubt it.
They're like Huey, Dewey, and Louie.
They'll stay forever as kids since it served the purpose.
Not unless it's conveniant for the plot.
