Background: Using biological evidence to define subtypes within the heterogeneous population with obsessive–compulsive disorder (OCD) is important for improving treatment response. Based on age at onset, OCD can be clustered into 2 groups, each of which is more homogeneous with respect to clinical and cognitive phenotype. However, the neural bases for these phenotypic differences need to be established to construct evidence-based homogeneous groups. 

Methods: We compared brain volumes, clinical symptoms, and neurocognitive function for 49 people with early-onset OCD and 52 with late-onset OCD (participants in both groups were unmedicated or drug-naïve), and 103 healthy controls. We performed regression analyses to examine group × volume interaction effects on clinical outcomes or neurocognitive function in people with OCD. 

Results: We observed larger volumes in the precentral, orbitofrontal, middle frontal, and middle temporal gyri in people with early-onset OCD compared to those with late-onset OCD. Poorer visuospatial construction in early-onset OCD was correlated with a larger left middle frontal gyrus volume. Impaired visuospatial memory in people with early-onset OCD and cognitive inflexibility in people with late-onset OCD were correlated with increased and decreased volume in the left middle frontal gyrus, respectively. We found group × volume interactions for obsessive–compulsive symptom scores in the left middle temporal gyrus of people with OCD. 

Limitations: Although we divided the subtypes using the commonly adopted criterion of age at onset, this criterion is still somewhat controversial. Conclusion: We provided the neural bases for clinical and neurocognitive differences to demonstrate that biological evidence underlies the distinctions between early- and late-onset OCD. This study suggests that different treatment options should be considered for the OCD subtypes, because their neurobiology differs and is related to distinct phenotypic profiles