I work in a residential facility which is home to 15 learning and (some) physically disabled adults. All are unique individuals in his/her own right. One can't help but love them, although sometimes they can be frustrating. A favorite quote from a former administrater is "Retarded means slow, NOT stupid!" How very true! many of our residents are very capable of teasing and having fun with us.
As far as this 'joke'...the only thing I chuckled at was the child's mispronunciation of 'retired'.