Words get taken and used this way. Idiot used to mean autistic. I wouldn't be surprised if there is already a word that is coming to the fore to replace the word retard or retarded as it has become a word too many people use to describe someone who is not actually retarded.
Not that I'm making a value judgement either way of course.
Mentally Retarded-when describing someone with a very low IQ.
Developmentally Disabled-which includes the low ID (below 70) and autism and CP.
Mentally Challenged- catch all for most conditions and considered to be appropriate for most conditions.