There is often a large time gap between caregivers' initial concerns and the diagnosis of autism spectrum disorder. The current study aimed to identify factors associated with missed or delayed autism spectrum disorder diagnoses among children in Colorado. In a surveillance-based sample of 8-year-old children with autism spectrum disorder (N=572), we examined differences between children who were identified with autism spectrum disorder by a community provider and/or were eligible for special education services under an autism eligibility (documented diagnosis) and children who were first identified with autism spectrum disorder through a systematic record review (newly identified). Compared to documented diagnosis children, newly identified children were more likely to be female, aggressive, and argumentative. They were less likely to have had a developmental regression, sleep abnormalities, or an autism screener or diagnostic measure in their records. Newly identified children also had a poorer quality of information in their records. Furthermore, among documented diagnosis children, variations in clinical presentations were associated with significantly different mean ages at autism spectrum disorder diagnosis; children who showed early delays, motor abnormalities, hyperactivity and attention deficits, and odd responses to sensory stimuli received a diagnosis much earlier than documented diagnosis children with other clinical presentations.