Bolton S, Warner J, Harriss E, et al. - In bipolar disorder (BD), a chronic mental health disorder with significant morbidity and mortality, age at onset (AAO) may aid in delineating more homogeneous subgroups of BD patients. Researchers herein systematically examined how BD age-at-onset subgroups should be defined. From Cochrane Central Register of Controlled Trials, PsycINFO, MEDLINE, Embase, CINAHL, Scopus, Proquest Dissertations and Theses, Google Scholar and BIOSIS Previews, they identified a total of 9,454 unique publications. Data analysis was performed including 21 of these publications (n = 22,981 BD participants). A trimodal AAO distribution (early-, mid- and late-onset subgroups), vs a bimodal distribution (early- vs. late-onset), is identified across a broader range of bipolar disorder diagnoses (BDI, BDII and schizoaffective disorder) and a greater number of patients (59% vs 6% of all participants—excluding cohort studies). Compelling evidence was thus generated suggesting that bipolar disorder onsets during early, mid or late life, with the majority (45%) of participants exhibiting an average age at onset of 17.3 years.