I think there was a time that the word, 'retarted'. Was socially accepted, until it had been misused like, today.
I know that in music, the word, 'ritard', means slow or to slow down. So it is not originally an insult. It is just kind of a broad word. It would be more polite to be detailed, though. If the special diagnoses is unknown, you could use a phrase like this of some sort: "he/she is challenged socially, mentally, physically, slow, dependent on, independent with limits."
I would have to say that I do not have a definite answer. But am sure I am very close.
retarded. Dictionary.com. WordNet® 3.0. Princeton University. http://dictionary.reference.com/browse/retarded (accessed: July 17, 2008).
A Thesaurus or a medical term resource may help.