

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.
Grinding n-grams might help