"A developmental disability is a long-term condition that significantly delays or limits mental or physical development and substantially interferes with such life activities as self-care, communication, learning, decision-making, capacity for independent living, and mobility. It is usually diagnosed before a person reaches age 22."

Missouri Department of Mental Health
Division of Mental Retardation and Developmental Disabilities (MRDD)
http://www.modmh.state.mo.us/mrdd/mrddfacts.html

"Persons with developmental disabilities may experience slow intellectual development in areas such as learning, reasoning, and memory. Developmental disabilities are caused by conditions that hinder or interfere with the developing brain before, during, or shortly after birth, or in early childhood. In most cases, the precise cause of the disability cannot be identified, although there are clear linkages to maternal infections during pregnancy, the consumption of toxic substances by the pregnant mother, premature childbirth, infections during infancy, and genetic defects. Some developmental disabilities are visible, such as Down Syndrome, while others may not be obvious."

Treasury Board Secretariat
http://www.tbs-sct.gc.ca/Pubs_pol/hrpubs/TB_852/DEVELOP_e.html