Transgender and gender diverse (TGD) people commonly report the following gender identity milestones: feeling different about their gender than expectations for their sex assigned at birth, identifying as TGD, living in their affirmed gender, and, for some, accessing gender-affirming medical care. We explored the average ages of reaching these milestones and variations across gender groups and generational cohorts. We also examined how gender groups, generational cohorts, and endorsement of reaching each of the milestones related to minority stress variables and mental health. This online study included 695 TGD individuals ages 16-73. Boomers+ and Generation X groups were more likely to identify as trans women compared to the younger generational cohorts, who were more varied in their identities. Trans women had later ages of starting to live in their affirmed gender and receiving gender affirming medical care compared to other gender groups. The Boomers+ cohort reported later ages for the milestones compared to other generational cohorts. And, finally, younger generational cohorts had higher levels of internalized stigma, anxiety, and depression, compared to the older cohorts. Gender congruence emerged as a consistent predictor of mental health in the full sample and within each generational cohort. There are important generational differences across identity milestones, minority stress, and mental health that need exploration in future longitudinal research. In addition, beyond the effects of milestone timing, reporting feelings of congruence with one’s gender identity is an important consideration for mental health.