The coming-out process for gay and bisexual men (GBM) involves crossing sexual identity development (SID) milestones: (1) self-awareness of sexual attraction to the same sex, (2) self-acceptance of an identity as gay or bisexual, (3) disclosure of this sexual identity to others, and (4) having sex with someone of the same sex. We examined trends in SID milestones by birth cohort in a 2015 U.S. national sample of GBM (n = 1,023). Birth cohort was independent of when men first felt sexually attracted to someone of the same sex (median age 11 to 12). However, with the exception of age of first same-sex attraction, older cohorts tended to pass other milestones at later ages than younger cohorts. Latent class analysis (LCA) of SID milestone patterns identified three subgroups. The majority (84%) began sexual identity development with same-sex attraction around the onset of puberty (i.e., around age 10) and progressed to self-identification, same-sex sexual activity, and coming out-in that order. The other two classes felt same-sex attraction during teen years (ages 12.5 to 18.0) but achieved the remaining SID milestones later in life. For 13% of men, this was during early adulthood; for 3% of men, this was in middle adulthood. Findings highlight the need to monitor ongoing generational differences in passing SID milestones.