BackgroundAgeing is an important factor in the development of mental health problems and their treatment. We assessed age trajectories of common mental disorders (CMDs) and psychotherapy utilization from adolescence to old age, and examined whether these trajectories were modified by time period or birth cohort effects.MethodBritish Household Panel Survey (BHPS) with an 18-year follow-up between 1991 and 2009 (n = 30 224 participants, aged 15–100 years, with an average 7.3 person-observations per person). CMDs were assessed with the 12-item version of the General Health Questionnaire (GHQ). Psychotherapy treatment utilization during the past year was self-reported by the participants. The modifying influences of time period and cohort effects were assessed in a cohort-sequential longitudinal setting.ResultsFollowing a moderate decrease after age 50, the prevalence of GHQ caseness increased steeply from age 75. This increase was more marked in the 2000s (GHQ prevalence increasing from 24% to 43%) than in the 1990s (from 22% to 34%). Psychotherapy utilization decreased after age 55, with no time period or cohort effects modifying the age trajectory. These ageing patterns were replicated in within-individual longitudinal analysis.ConclusionsOld age is associated with higher risk of CMDs, and this association has become more marked during the past two decades. Ageing is also associated with an increasing discrepancy between prevalence of mental disorders and provision of treatment, as indicated by lower use of psychotherapy in older individuals.