BackgroundThe obsessive-compulsive disorder （OCD） features complexity in etiological factors and high heterogeneity in clinical manifestations. OCD patients with different ages of onset vary in clinical symptoms and etiology. However， current studies on inpatients with early- and late-onset OCD are limited. ObjectiveTo explore the differences in clinical characteristics between early- and late-onset OCD inpatients as well as the factors affecting the onset age of OCD， so as to provide references for early screening and treatment of OCD patients. MethodsThis study was based on collected medical records of 540 patients with OCD who received inpatient treatments at the Affiliated Brain Hospital of Nanjing Medical University between March 2012 and March 2023. Patients with onset age above 18 were placed into early-onset group （n=310） and the others into late-onset group （n=230）. Then differences in demographic data and clinical symptoms between two groups of patients were compared. Binary logistic regression was used to analyze the factors that affect the onset age of OCD. ResultsObserving the demographic data， there were significant differences between the two groups in the results in gender， marital status， family history of mental illness， ratio of comorbidities with other mental illnesses， occupational composition， education level and types of obsessive-compulsive symptoms （χ²=22.302、170.556， 9.224， 13.624， 242.277， 59.791， 7.231， P<0.05 or 0.01）. Also， the results in ages of onset and hospitalization between two groups were significantly different （Z=-19.915， 16.831， P<0.01）. In terms of clinical symptoms， the early onset group had a higher proportion of symptoms including obsessive thinking （χ²=11.998， P<0.05）， ordering （χ²=7.731， P<0.05） and rituals （χ²=7.714， P<0.05）， while the proportion of obsessive checking （χ²=8.204， P<0.05） and washing （χ²=7.506， P<0.05） symptoms were relatively low. In terms of risk factors， there were several independent risk factors that influence the onset age of OCD inpatients， including comorbid neurodevelopmental disorder， comorbid affective disorder， family history of schizophrenia and family history of affective disorder （OR=19.587， 1.830， 3.065， 4.431， P<0.05）. Among them， comorbid neurodevelopmental disorder was the core influencing factor， and female gender was a protective factor for early-onset patients （OR=0.417， P<0.01）. ConclusionThere are differences in demographic data and clinical symptom characteristics between early- and late-onset OCD inpatients， and comorbid neurodevelopmental disorder plays as a core risk factor affecting the onset age of OCD inpatients. ［Funded by Jiangsu Province Key Research and Development Plan for Social Development Special Project（number， BE2021616） ； Jiangsu Province Social Development General Project （number， BE2022678）； Key Project of Nanjing Medical Science and Technology Development Fund （number， ZKX20029）］