Some kids can. Some kids definitely learn only one language really well and struggle with a second indefinitely.

I'm sure there's some cut-off and a way to do it right and so-forth. But are the surviving native speak of an obscure language in some rural village going to suss that out.

Plus a lot of kids who a language just when they're young go to forget. I remember, growing up, a friend who could understand but not speak Urdu. He wasn't going to perpetuate the language.