Ben is mentally retarded. It is okay to use those words in that context. There is a move to use different words like cognitively delayed and intellectually disabled because of the negative connotation mentally retarded has been given over the past several decades.
I think most people who use the word retarded or retard do so because it is something they have always said. Sometimes a bit of education and an effort to break a bad habit is all it takes. So spread the word, stop using the word in the wrong context.
PS -This link was sent to me and I thought I would add to this post. USA Today Story