Symptoms of ASD most commonly are recognized in the 2nd year of life but may be present earlier. Symptoms in children with the least severe phenotypes of ASD may not be apparent to parents or teachers until 4 to 6 years of age or later.
Approximately 2/3rd of patients with ASD present with lack of acquisition of communication skills during the first 2 years of life. Approximately 1/4th to 1/3rd of children with ASD achieve early language milestones, but have regression of language, communication, and/or social skills between 15 to 24 months of age. The regression of skills can be gradual or sudden and may occur in the context of pre-existing developmental delays or atypical development.