First of all we need to understand that Mental Retardation is not a disease likefever, cancer, hepatitis etc.
It is a condition. And when it is not a disease that means we do not need to provide a cure for it. Thus we need to focus all of our attention and energy for habilitation of the person affected in the society.
Mental retardation is a delay or slowness in the mental development of a person. It is a condition and not a disease. Person with this condition will have less mental ability or intelligence than others of his age. So a child who is ten years old may behave like a child of 3-5years.
Technically, to call a person mentally retarded we see these things:
1) Intelligence Quotient: It should be below 70.
2) Behaviour: The child is lacking in appropriate behavior as per his or her age group.
3) Age : Child should be below 18 years of age.
The prevalence rate of MR is 2-3% of total population. About 30 million people in India are Mentally Retarded of these about 15million are below the age of 10.