Many immigrant kids grow up understanding a language fluently, but unable to speak it. This is known as being a passive speaker, as far as I know.
If they decide later to actively begin learning to speak the language they understand, will they be able to become as fluent as a native speaker? Presumably they have some advantage over those who don't know the language in any way at all.
I'm also interested in whether or not they can have a native level pronunciation of their language when actively speaking it, apart from grammar.
I'm particularly interested in whether or not a teenager (14-17) who had a strong interest in their language would be able to achieve this, since it is well known that older learners cannot learn a new language as well as a younger person could.