34 C.F.R. §300.7. Child with a disability.

(c) Definitions of disability terms.

(1)(i) Autism means a developmental disability significantly affecting verbal and nonverbal communication and social interaction, generally evident before age 3, that adversely affects a child's educational performance. Other characteristics often associated with autism are engagement in repetitive activities and stereotyped  movements, resistance to environmental change or change in daily routines, and unusual responses to sensory experiences. The term does not apply if a child's educational performance is adversely affected primarily because the child has an emotional disturbance, as defined in paragraph (b)(4) of this section.
(ii) A child who manifests the characteristics of ``autism'' after age 3 could be diagnosed as having ``autism'' if the criteria in paragraph (c)(1)(i) of this section are satisfied.