something that is "stunted" from birth IS a defect. i dont see why you're trying to redefine things. who cares?
you can call them, retarded, stupid, mentally ill, defective.... theres many terms, some more polite than others. either way, it all boils down to the same conditions and everyone will still know what youre talking about. Whether you call them mentally retarded or mentally ill is no different and shouldnt even be argued about. its just how the language works. its like calling someone a fag when theyre not really gay. it is the wrong word to use, but everyone still knows what you mean so it doesnt matter