Early diagnosis of autism spectrum disorder is challenging due to its phenotypic and etiological heterogeneity, but critical for implementation of early interventions. We examined the age of autism spectrum disorder diagnosis in a sample of 554 children and adolescents enrolled in the ELENA cohort study in France with regard to the influences of child clinical characteristics, family antecedents, and socio-economic factors. The mean age of diagnosis was 4.9 years (±2.8 years) with prediction of diagnosis before 3 years of age related to co-occurring intellectual disability, higher autism spectrum disorder symptom severity, and lower communicative abilities. Children in low socio-economic status families tended to have earlier autism spectrum disorder diagnosis compared to those in high socio-economic status families, but they also had greater severity of intellectual impairment. The age of autism spectrum disorder diagnosis was not associated with the presence of an older sibling with autism spectrum disorder. The trend for an inverse relationship between socio-economic status and age of diagnosis suggests equity in the current diagnostic services in France where health coverage is universal and free. Nonetheless, better screening of more subtle/less severe forms of autism spectrum disorder is also needed, as well as further assessment of the link between co-occurrence of autism spectrum disorder and severity of intellectual impairment in lower socio-economic status families. Lay Abstract Autism spectrum disorder is an early onset neurodevelopmental disorder and diagnosis can be made as early as 18 months of age. Early diagnosis of autism spectrum disorder is critical as it leads to early intervention. Age of autism spectrum disorder diagnosis has been linked to the child profile as autism spectrum disorder is characterized by strong heterogeneity, but is also influenced by socio-economic factors. There is paucity of data on age of diagnosis of autism spectrum disorder in France. We therefore examined the age of autism spectrum disorder diagnosis in 554 children and adolescents enrolled in the ELENA cohort study with respect to the influences of child profile, family antecedents, and socio-economic factors. The mean age of diagnosis was 4.9 years (±2.8 years). Early diagnosis, before 3 years of age, was related to the co-occurrence of intellectual disability, higher autism spectrum disorder symptom severity, and lower communicative abilities. Children in low socio-economic status families tended to have an earlier diagnosis, but these children also had greater degree of intellectual impairment compared to children in high socio-economic status families. The age of autism spectrum disorder diagnosis was not associated with the presence of an older sibling with autism spectrum disorder. The observed current trend of an inverse relationship between socio-economic status and age of diagnosis of autism spectrum disorder suggests equitable access to autism spectrum disorder services in France where health coverage is universal and free. Better screening of more subtle/less severe forms of autism spectrum disorder is needed, as well as further assessment of the link between the co-occurrence of autism spectrum disorder and intellectual impairment in children in lower socio-economic status families.