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. Timely detection is an issue of paramount importance in the care of children with autism spectrum disorder. Whether the delayed autism spectrum disorder diagnosis can be explained by children's clinical presentations and socio-familial status in China is a question to be addressed. We investigated 1235 autism spectrum disorder children from 132 rehabilitation organisations in Shenzhen, China. These children were found to have a mean age of diagnosis of 31.4 ± 12.7 months and a median age of diagnosis of 30.0 months. The majority of these children were able to receive their diagnosis during toddlerhood. However, about one in six were not diagnosed until they entered preschool or later, thus missing the golden window of opportunity for early intervention. The age of diagnosis was likely to be late if the children were older, were less severe and presented with no intellectual impairment. The odds of having a delayed autism spectrum disorder diagnosis were more than 9 times higher among migrant autism spectrum disorder children than among those with local household registrations, thus underscoring the importance of identifying culturally sensitive socio-economic determinants in autism spectrum disorder detection, as these factors are likely to affect the quality of life of many autism spectrum disorder children and their families. [ABSTRACT FROM AUTHOR]