mentally handicapped 
adjective dated or offensive
(Of a person) having very limited intellectual functions: a mentally handicapped child [as plural noun]: she works with the mentally handicapped...
- Children were excluded if they were diagnosed as mentally handicapped or if they had a pervasive developmental disorder.
- The health care needs of severely mentally handicapped children are being neglected because of an over-emphasis on education, their parents have claimed.
- But there have been heart-rending stories, such as an elderly woman with no support forced to stay home to look after her mentally handicapped daughter.