I’m bilingual since a child but didn’t really use one of the languages much after age 12 to about 30. When I do speak the second language I can fool locals into thinking it’s my first language, but it takes exactly 1 mistake or mispronunciation and they ask “oh are you actually English?”. The bar is that that high for passing native fluency in another language, if you somehow could fake the accent perfectly as well (there is absolutely no way).