I'm not sure it's that cut and dry, nor that any learning needs to be "undone". The study itself (referenced in this article) is co-authored by Pinker, and these ideas take time and research to develop.
http://www.asian-efl-journal.com/March_2002_EBook_editions.p...
^ Basically the above study, sponsored by the NIH, found that when you correct for all relevant factors the critical period signal not only disappears but becomes anti-correlated: kids have a HARDER time in all aspects of language learning than adults, when the same level of interest and effort is applied.
That's, IMO, an unwarranted (and, taken strictly, almost implausible) assumption. (Especially on the interest side.)