The timing of autism spectrum disorder detection in research and clinical practice is characterised by substantial heterogeneity ranging from infancy to school age. In this study, we examined the age of diagnosis and its associated individual and socio-familial factors in Chinese children diagnosed with autism spectrum disorder. A sample of 1235 autism spectrum disorder children from 132 rehabilitation organisations in Shenzhen was investigated in 2021. We found a mean age of diagnosis of 31.4 ± 12.7 months and a median of 30.0 months. Although 71.7% of the children received their diagnosis between 25 and 36 months of age, and 13.3% were diagnosed early (?24 months), 15.1% did not have their symptoms detected until preschool or later years (>36 months). The diagnosis was likely delayed (>36 months) if the children were older, were less severe and presented with no intellectual impairment. The odds of delayed autism spectrum disorder diagnosis were more than 9 times higher among migrant autism spectrum disorder children than those with local household registrations. The study underscores the importance of identifying culturally sensitive socio-economic determinants in autism spectrum disorder detection in addition to clinical factors, as the former are likely to affect the quality of life of many autism spectrum disorder children and their families.