OBJECTIVE Although bipolar disorder (BD) is a common recurrent condition with highly heterogeneous illness course, data are limited regarding clinical implications of interactions between gender and onset age. We assessed relationships between onset age and demographic/illness characteristics among BD patients stratified by gender. METHODS Demographic and unfavorable illness characteristics, descriptive traits, and clinical correlates were compared in 502 patients from Stanford University BD Clinic patients enrolled in the Systematic Treatment Enhancement Program for BD between 2000 and 2011, stratified by gender, across pre-, peri-, and post-pubertal (<12, 13-16, and >17 years, respectively) onset-age subgroups. RESULTS Among 502 BD patients, 58.2% were female, of whom 21.9% had pre-pubertal, 30.7% peri-pubertal, and 47.4% post-pubertal onset. Between genders, although demographics, descriptive characteristics, and most clinical correlates were statistically similar, there were distinctive onset-age related patterns of unfavorable illness characteristics. Among females, rates of 6/8 primary unfavorable illness characteristics were significantly higher in pre-pubertal and peri-pubertal compared to post-pubertal onset patients. However, among males, rates of only 3/8 unfavorable illness characteristics were significantly higher in only pre-pubertal versus post-pubertal onset patients, and none between peri-pubertal versus post-pubertal onset patients. LIMITATIONS Caucasian, insured, suburban, American specialty clinic-referred sample limits generalizability, onset age based on retrospective recall. DISCUSSION We describe different phenotypic presentations across age at illness onset groups according to gender. Among females and males, peri-pubertal and post-pubertal onset age groups were more different and more similar, respectively. Further investigation is warranted to assess implications of gender-by-onset-age interactions to more accurately delineate distinctive BD phenotypes.