Disparities in mental health and bullying between SGM youth and their heterosexual, cisgender peers are well-established. There remain questions about whether the onset and progression of these disparities differ across adolescence—knowledge critical for screening, prevention, and intervention. To address this, the current study estimates age-based patterns of homophobic bullying, gender-based bullying, and mental health across groups of adolescents defined by sexual orientation and gender identity (SOGI). Data are from the 2013–2015 cycle of the California Healthy Kids Survey ( n  = 728,204). We estimated the age-specific prevalence rates of past-year homophobic bullying, gender-based bullying, and depressive symptoms using three- and two-way interactions by (1) age, sex, and sexual identity and (2) age and gender identity, respectively. We also tested how adjustments for bias-based bullying alter predicted prevalence rates of past-year mental health symptoms. Results showed that SOGI differences in homophobic bullying, gender-based bullying, and mental health were already present among youth aged 11 and younger. SOGI differences by age were attenuated when adjusting models for homophobic and gender-based bullying, particularly among transgender youth. SOGI-related bias-based bullying and mental health disparities were present early and generally persisted throughout adolescence. Strategies that prevent exposure to homophobic and gender-based bullying would significantly reduce SOGI-related disparities in mental health across adolescence.