Let them vote. Unless everyone in the entire world can agree on the definition of a profound mental retardation that impairs judgement to such an extent as to not be able to fairly judge candidates, that should be the way it should be.
To take the example of Down's syndrome: some people with this disorder live fairly normal lives, with jobs, pensions and sometimes even partners and children. Yet others are totally reliant on other people. Where do you draw the line? Answer: you can't. Nothing is black and white.
The same goes for Autism. That covers a wide range of disorders, from those such as Asperger's, which only midly affect judgement (and some would argue not even in a bad way, though this is debatable), to disorders which are so severe that the sufferer loses the ability to speak. Where do you draw the line?
Also, remember that someone would have to set guidelines on who is and who is not allowed to vote. That could be manipulated by some people with unscrupulous motives, and who wants to take that risk?