Adolescent sexual development is not the same across the board, and can be seen as both normative as well as risky behavior, depending on factors such as age of onset and number of partners. This longitudinal study identified developmental trajectories of annual number of sexual partners from adolescence to emerging adulthood and their antecedents. From the ages of 16–22, 332 participants (60.8 % females) annually reported their number of sexual partners. Measures of adolescent and family characteristics taken at ages 13–15 were used as predictors. Group-based modeling identified four trajectory groups: the abstainers group (9.1 %), low-increasing group (30.6 %), medium-increasing group (53.0 %), and multiple-partners group (7.3 %). Multinomial logistical regressions indicated that better social competence increased chances of belonging to the multiple-partners group as compared to the abstainers and low-increasing groups, and more substance use predicted membership in the multiple-partners group as compared to the abstainers group. Moreover, females were overrepresented in the low-increasing and medium-increasing groups. These results imply a greater diversity in sexual development than proposed in the literature and highlight the importance of identifying the strengths and competencies associated with healthy sexual development along with the early risk factors.