Although paediatric obsessive--compulsive disorder (OCD) is increasingly recognized as a putative developmental subtype of the disorder, it remains uncertain as to whether additional subtyping by age at onset in childhood or adolescence is warranted. Subjects included children and adolescents meeting DSM-III-R and DSM-IV criteria for OCD referred to a specialized OCD clinic. All youth were systematically evaluated with structured diagnostic interviews and clinical assessment by an OCD expert. Irrespective of current age, an earlier age at onset predicted increased risk for attention deficit hyperactivity disorder, simple phobia, agoraphobia and multiple anxiety disorders. In contrast, mood and psychotic disorders were associated with chronological age and were more prevalent in older subjects. Tourette's disorder showed associations with both chronological age and age at onset. Chronological age and age at onset predicted different patterns of comorbidity and dysfunction in children and adolescents with OCD. Considering the heterogeneity of OCD, age at onset may help identify meaningful developmental subtypes of the disorder beyond chronological age.