It would be useful to distinguish kinds and levels of bilinguality. Is it conversational, or could the person read a novel or history book in either language? Or write a polished essay?

In any case, given that one learns a language well almost only by living where that language is spoken, and/or receiving education in that language, the key to true bilinguality is a strategic set of moves between linguistic regions across the child’s developmental period.

I am reminded of my childhood friend who came from Hungary to the US as a child and lived in a very Hungarian environment, his parents speaking only Hungarian, going to Hungarian schools and cultural events on the weekends, etc. He told me though, when he finally returned to Hungary 40 years later, people listening to him looked at him like he was a Martian, and he felt like one too.