“a person who has spoken the language in question from earliest childhood”

And this makes sense, because “native” as an adjective means:

“associated with the place or circumstances of a person's birth”

So native speaker literally means a speaker by birth. Where are you getting the “fluent only by immersion” definition?

Are you instead talking about “native fluency,” which is typically used to mean fluency at the level of a native speaker, which is technically achievable by anyone (though realistically impossible after a certain age)?