very interesting. I agree with you that it's the phonemes that is the problem. As one ages, one's primary language phonemes sound more and more "normal", and subtlety that do not exist in one's primary language is "tossed out" by the brain.

I learned to speak English when I was 10, and I have no issue with the differences between Chinese and English. But I was exposed to French only a little bit later (12) and I am completely hopeless at it. The grammar, spelling and vocabulary are the easiest parts; I just can't seem to produce the same sounds.