Actually I did something similar - learned to speak French with no perceptible foreign accent at age 17 (unfortunately my abilities have decayed in the interven

Actually I did something similar - learned to speak French with no perceptible foreign accent at age 17 (unfortunately my abilities have decayed in the intervening years, so I’m sure now I’d sound American, but anyway…) However, accent is only one part of language. I certainly didn’t have the vocabulary, reading speed, writing elegance, etc. of (educated) native speakers.