The Mental Retardation cutoffs, per DSM-IV:

       Mental Retardation          IQ               Equivalent
       Mild                              50-55 - 70        6th grade
       Moderate                     35-40 - 50-55   1st grade
       Severe                          20-25 - 35-40   Requires constant supervision
       Profound                       below 20-25     Requires constant care
 

Mentally retarded children:

• develop individual personalities and preferences;
• are no more likely to commit a crime than their peers;
• do not have difficulty bonding as children;
• ARE at increased risk for development of another mental disorder.
• do NOT necessarily have perceptual disturbances, violent or sexual ideation, or delusional thinking (they may, but it would be acccounted for by another diagnosis, not their mental retardation);.
• tend to be very concrete in their thinking and reasoning.
• go through milestones in normal ORDER, but often plateau at a lower than normal level.

        Mental retardation has an onset before age 18 (normal cognitive functioning followed by a cognitive decline is dementia, not mental retardation).

References:

DSM-IV, pages 39-46.
Kaplan, 1025-1041.