No, Mentally retarded persons are not mentally ill. The mentally retarded persons are just slow in their development. Therefore, they are dull and slow in understanding and have difficulty in learning various skills needed for daily living. Usually they have problems in speech. Some of them can be educated up to 5th class while the others cannot reach even this level.The mentally ill on the other hand have normal development. Mental illness can occur at any age even among the highly qualified people. Mental illness can be generally cured..