OBJECTIVE: Many studies suggest that age at onset (AAO) is an important factor for clinically differentiating patients with juvenile and adult onset of obsessive-compulsive disorder (OCD). The present international study aimed to assess the prevalence of different AAO groups and compare related socio-demographic and clinical features in a large sample of OCD patients. METHODS: A total of 431 OCD outpatients, participating in the ICOCS network, were first categorised in groups with childhood (≤12 years), adolescent (13-17 years) and adult-onset (≥18 years), then in pre-adult and adult onset (≥18 years) and their socio-demographic and clinical features compared. RESULTS: Twenty-one percent (n = 92) of the sample reported childhood onset, 36% (n = 155) adolescent onset, and 43% (n = 184) adult onset. Patients with adult onset showed a significantly higher proportion of females compared with the other subgroups (χ(2 )=( )10.9, p