The federal definition of developmental disabilities covers persons whose disability occurs before age 22 and includes a mental or physical impairment or a combination of both.  There must be a substantial limitation in three or more of these major life areas: self care; expressive or receptive language; mobility; capacity for independent living; economic self-sufficiency; or self-direction.

In California law, a developmental disability is more narrowly defined as occurring before the age of 18 and includes specific categories of eligible conditions: intellectual disabilities, epilepsy, cerebral palsy, autism, and “conditions requiring services similar to those required for persons with mental retardation.” California law required that the individual be substantially handicapped by the disability. In March 2004,  new regulations took effect that define substantial disability as significant functional limitations in three or more of the major life activities contained in the federal definition above.