Transgender and gender nonconforming (TGNC) people experience high rates of minority stress and associated risk for negative health outcomes. However, during the last years, significant positive socio-cultural changes have happened, and younger cohorts of TGNC individuals are having diverse experiences compared to older cohorts. By integrating the minority stress theory and the life course perspective, this cross-sectional, web-based study aimed to explore in 197 Italian TGNC people aged 18 to 54 years (M = 29.82, SD = 9.64) whether the average ages of gender identity milestones (i.e., first insights about being TGNC, self-labeling as a TGNC person, and coming out), minority stress, and mental health vary among three generational cohorts (i.e., Generation Z, Millennials, and Generation X). Compared with older cohorts, younger participants: (a) were more likely to be in the trans-masculine spectrum; (b) self-labeled as TGNC and came out earlier; (c) had more negative expectations and lower levels of disclosure; and (d) had higher levels of mental health problems. No generational differences related to first insights about being TGNC and distal minority stressors were found. Furthermore, compared with binary individuals, participants with a non-binary identity: (a) reported later ages for the gender identity milestones; (b) had higher negative expectations; and (c) had higher levels of mental health problems. Overall, our findings indicated that changes in the social environments have a limited impact on stigmatization processes and mental health of Italian TGNC people.