Factors that have a bearing on adolescent pregnancies have been widely documented worldwide. However, rarely has the adolescents’ own perception on the ideal timing to transit into adulthood-related events, such as family formation, been considered. Neither has it been widely investigated if these aspirations are linked to the risk of early pregnancies. Using data from a cross-sectional study with a sample of adolescents in Mexico City and Morelos, Mexico, we explored expectations regarding the ideal timing for family formation (marriage and childbearing) and their link to sexual behavior. We surveyed 3,332 adolescents in eight middle schools and high schools. Four types of aspirations were identified: (a) early plan: desire to marry at age 21 and have children at 22; (b) middle plan: desire to marry and have children at ages 25 and 26 respectively; (c) late plan: desire for a stable partner at age 29 and children at 31; and (d) non-traditional plan: adolescents that do not want to marry and/or have children. Those adolescents that aspire to postpone family formation particularly until 29 years of age (late plan) are more likely to report condom use at first intercourse and to have fewer sexual relations during all their life. Adolescents’ expectations about their life in the future affect their present sexual and reproductive decisions. Identifying their aspirations should enhance the design of more viable and successful policy strategies to prevent adolescent pregnancies.