1) Anecdotal evidence doesn't really mean anything. Everyone develops slightly differently - 11 years isn't a hard wired rule. These are rough averages. I expect that in the data there were some nine year olds that had difficulty assimilating, and some 13 year olds that had no trouble.
2) What the early exposure does is for example train your brain on distinguishing and producing phonemes which would otherwise literally be imperceptible otherwise.
3) French, Spanish, and Italian are very similar languages, in structure, idioms, and phonetics. It is not uncommon for adult learners of these languages coming from related backgrounds to develop a near-native accent within a realistic study regimen. This is because there are not that many perceptual hurdles between these European languages - if you've learnt one of these languages as a child, you have most of the mental machinery necessary to handle a native accent in one of the others. IIRC these studies are often more about Vietnamese or Chinese kids learning English or vice versa, where the problems are much, much greater and the prior of adult success very low.
NPR reports on a study giving volprioc acid to adults and training them on pitch (singing):
Brain plasticity is useful for a whole lot more than learning pitch. As the article notes it would be invaluable for training one's ear to pick up sounds of foreign languages, but also it seems reasonable to this commentator that high levels of plasticity during rationality training or other forms of self-development would result in more transformative results.