This research examined endorsement and timing of sexual orientation developmental milestones. Participants were 1,235 females and 398 males from the Growing Up Today Study, ages 22 to 29 years, who endorsed a sexual minority orientation (lesbian/gay, bisexual, mostly heterosexual) or reported same-gender sexual behavior (heterosexual with same-gender sexual experience). An online survey measured current sexual orientation and endorsement and timing (age first experienced) of five sexual orientation developmental milestones: same-gender attractions, other-gender attractions, same-gender sexual experience, other-gender sexual experience, and sexual minority identification. Descriptive analyses and analyses to test for gender and sexual orientation group differences were conducted. Results indicated that women were more likely than men to endorse same-gender attraction, other-gender attraction, and other-gender sexual experience, with the most gender differences in endorsement among mostly heterosexuals and heterosexuals with same-gender sexual experience. In general, men reached milestones earlier than women, with the most gender differences in timing among lesbian and gay individuals and heterosexuals with same-gender sexual experience. Results suggest that the three sexual minority developmental milestones may best characterize the experiences of lesbians, gay males, and female and male bisexuals. More research is needed to understand sexual orientation development among mostly heterosexuals and heterosexuals with same-gender sexual experience.