Intellectual disability is a disability characterized by significant limitations in both intellectual functioning and inadaptive behavior, which covers many everyday social and practical skills. This disability originates before the age of 18.

Intellectual Functioning

Intellectual functioning—also called intelligence—refers to general mental capacity, such as learning, reasoning, problem solving, and so on.

One way to measure intellectual functioning is an IQ test. Generally, an IQ test score of around 70 or as high as 75 indicates a limitation in intellectual functioning.

Adaptive Behavior

Adaptive behavior is the collection of conceptual, social, and practical skills that are learned and performed by people in their everyday lives. 

• Conceptual skills—language and literacy; money, time, and number concepts; and self-direction.
• Social skills—interpersonal skills, social responsibility, self-esteem, gullibility, naïveté (i.e., wariness), social problem solving, and the ability to follow rules/obey laws and to avoid being victimized.
• Practical skills—activities of daily living (personal care), occupational skills, healthcare, travel/transportation, schedules/routines, safety, use of money, use of the telephone.

Standardized tests can also determine limitations in adaptive behavior.

Age of Onset

This condition is one of serveral developmental disabilities—that is, there is evidence of the disability during the developmental period, which in the US is operationalized as before the age of 18.

Additional Considerations

But in defining and assessing intellectual disability, the AAIDD stresses that additional factors must be taken into account, such as the community environment typical of the individual’s peers and culture. Professionals should also consider linguistic diversity and cultural differences in the way people communicate, move, and behave.

Finally, assessments must also assume that limitations in individuals often coexist with strengths, and that a person’s level of life functioning will improve if appropriate personalized supports are provided over a sustained period.
 

Only on the basis of such many-sided evaluations can professionals determine whether an individual has intellectual disability and tailor individualized support plans.

Source: http://aaidd.org/intellectual-disability/definition#.V5o66fnR_VQ