Age at onset of depression is linked to clinical presentation and outcome, but its relevance for electroconvulsive therapy (ECT) remains unclear. We examined whether age at onset of first depressive episode requiring hospitalization predicts ECT response and remission, and whether onset age reflects distinct clinical profiles.

A nationwide register-based study, including 14,141 patients who received an index ECT series for a depressive episode in Sweden during 2008–2021, with outcome data in the Swedish Quality Register for ECT. Response was “very much/much improved” on the Clinical Global Impression – Improvement scale, and remission was a post-treatment Montgomery-Åsberg Depression Rating Scale-Self report score of 10 or less. Hierarchical logistic regression and causal mediation analyses examined associations of age at onset (early, mid, late), chronological age, baseline severity, psychotic symptoms, and somatic and psychiatric comorbidity with outcome and their mediating role.

Response and remission rates were highest in late-onset depression (84.9%, 60.2%) and lowest in early-onset depression (64.6%, 26.3%). Later onset and older age were independently associated with higher odds of both outcomes, but the onset-age association attenuated substantially after adjustment. The early-onset disadvantage operated mainly through psychiatric comorbidity and treatment complexity, and the late-onset advantage was driven by psychotic features. Associations of age, male sex, and psychotic symptoms with response differed by onset group.

Age of onset of first depressive episode requiring hospitalization was associated with clinical profiles and ECT outcomes, independent of chronological age, suggesting early- and late-onset depression are partly distinct subtypes relevant to treatment planning.