We were talking accents and ability to learn new languages or skills here, not birth defects - the one doesn't have anything to do with the other unless it is a form of direct speech impediment....
I mentioned the birth defect as simply an example of how they discovered there is a critical developmental period for vision.
As far as critical developmental periods for people without birth defects, there are many accounts of feral children that were never able to learn speech. There really is evidence for a critical developmental period for speech functions.
