[adastra22]

I have, multiple times. I’ve also raised bilingual kids. Adults fare vastly better (an order of magnitude) than kids, when compared on a basis of hours invested. The thing is, kids are exposed to a language learning environment 24/7/365 with few other responsibilities or a mother tongue to fall back on to express themselves.

If you were able to put in the same immersion effort as is forced upon a kid, you’d be fluent in less than a year and native-level a few years later, for most languages.