YouTube anecdotes, but here is Gabriel Wyner: https://www.youtube.com/watch?v=iBMfg4WkKL8 describing his experience of learning nothing much linguistically in High School or College, but then joining immersion based courses at an adult and making significant progress in a few weeks.

and here: is Chris Lonsdale saying adults can learn useful amounts of spoken Chinese in 6 months, with immersion and being able to watch mouths closely and see how speakers move their moouths to pronounce things (IIRC): https://www.youtube.com/watch?v=d0yGdNEWdn0

and there's a bunch more; https://www.youtube.com/watch?v=-WLHr1_EVtQ and so on