I’m from Vietnam. I’ve been in the UK for 10 years now. When I met my English husband 13 years ago at 19 I knew 0 English. We communicated using machine translation. So that’s when I started learning English. Fast forward to present day after immersion, living in an English speaking country, formal study, etc. and I’d say my writing and listening (understanding) are good, but my speaking and reading are still bad. I kind of gave up on trying to become fluent at this point.
For near-native fluency, there is an age cap at around 10 years. It’s much harder for adults, as their critical learning period is closed: https://news.mit.edu/2018/cognitive-scientists-define-critical-period-learning-language-0501
However there is evidence that psychedelics can open up critical periods for social learning (https://www.nature.com/articles/s41586-023-06204-3) and ongoing research about other critical periods, language learning being one of them.