Abstract
Autism Spectrum Disorder (ASD) is an early onset neurodevelopmental disorder in which the first signs generally emerge at approximately 12 months of age but its diagnosis is feasible only from the age of 18 months. According to the literature, the average age of diagnosis ranges from 2.7 to 7.2 years, which raises the question of factors associated with early diagnosis as a condition for early intervention. In this systematic review, we aim to identify clinical, social, and environmental factors associated with the age at which the diagnosis of ASD is confirmed in children. A literature search was performed in the Pubmed, Web of Sciences, PsycInfo, and Cochrane databases. Among the 530 publications identified, 50 were selected according to the inclusion criteria. This review focuses on studies conducted in 21 countries using data collected over a period from 1987 to 2017. These studies were published before December 31st, 2019. The results suggest that the diagnosis of ASD occurs earlier if there is a delay in social communication or the presence of intellectual disability. There is a low level of evidence concerning associations between the age at diagnosis and sex, race, parental education, or socioeconomic status and accessibility to health care. Further studies using large and well-characterized data sets are needed to simultaneously explore clinical and socio-environmental factors involved in early diagnosis.