Obsessive-compulsive disorder (OCD) has a bimodal age of onset and a range of treatment outcomes. Although most of the studies carried out so far have considered childhood and adult forms of OCD as the same disorder, more recent data have suggested that OCD children, as well as adults with an early onset of their obsessive-compulsive symptoms, may represent a distinct subgroup. This review briefly summarizes the most common clinical characteristics of the OCD presentation in children and adolescents, shows data reinforcing the idea that age of onset may be an important distinguishing feature and discusses the importance of a systematic assessment of age of onset for identifying more homogeneous subgroups of OCD patients.