That’s a very subjective window.

I know a guy who’s lived in my country for more than 12 years, and still can’t really speak but the most basic sentences, however, it is understandable, because his native language is English, and people like having the chance to practice their English. So despite being immersed and even needing the language, he just hasn’t really gotten around to learning it.

That’s the difference between language acquisition and learning one as an adult; motivation.

If you don’t have any, you’re not gonna learn. Kids can lack it totally, and they’ll still learn to speak, just as much as they’ll learn to walk.