I've honestly no idea, but I think the OP begs the question of whether acquiring a native accent when learning a foreign language is desirable in the first place.
I agree that children are comparatively better at learning accents than adults, yet on the other hand they're also more vulnerable to poor or inadequate teaching. Myself, having originally started learning English in Norway as a ten-year-old around 1990, I think I'm a case in point.
In the first couple years, there was a huge emphasis on pronunciation (at the expense of, for example, acquiring a decent vocabulary): the ideal at the time was RP - as if we were all going to be news presenters for the BBC when we grew up - however after school we'd go home and watch hour after hour of hip hop videos and Hollywood action films, with the result that most of us eventually sounded like an absurd cross-breed between 2Pac and the Prince of Wales. As an adult, I developed a slightly bland, but useful transatlantic accent which seems to work in most Anglophone nations (although no native speaker of English would take me for one).
I also have some experience in teaching Norwegian to adult immigrants, in which case my fellow teachers and I paid as little attention to accent as practically possible, at least in the beginner's class. Of course, if someone is incapable of coherent pronunciation you have to work on it, but fact is, as you say, that most will be incapable of shaking the foreign accent in any case. To complicate the issue further, Norwegian has very strong regional accents, so it would be a question of which to teach them: in effect, therefore, we made them concentrate their efforts on fluency, vocabulary and idioms, and I generally still think that's the best approach.
W.