The Reason That 'Onset Prior to Age 18' Is Part

The reason that 'onset prior to age 18' is part of the DSM-5 diagnostic criteria for intellectual disability is that

A) the diagnosis indicates that the individual was affected during the developmental period.
B) the diagnosis is not intended to include individuals who have suffered any form of brain damage.
C) the diagnosis has legal implications for children.
D) this diagnosis is reserved for children; other diagnostic terms are applied once the individual reaches the age of 18.