This study puts forth three criteria for assessing the extent to which the timing of sexual, marital, and reproductive transitions among male and female adolescents could be considered “too young”: (1) the physiological maturation of the body; (2) the cognitive capacity for making safe, informed, and voluntary decisions; and (3) institutionalized concepts of “old enough” for consent to sexual intercourse and marriage as reflected in legal frameworks and international standards. Expansion of the age grouping of adolescence is proposed, from the customary 15–19 into three age categories—early adolescence (ages 10–14, or 10–11 and 12–14), middle adolescence (15–17), and late adolescence (18–19)—to better capture the age-specific variations in the trajectories of male and female sexual, marital, and reproductive events. An application of the three adolescent development criteria to the timing of transitions observed in Demographic and Health Surveys in 64 developing countries leads to the conclusion that boys and girls aged 14 and younger are universally “too young” to make safe and consensual transitions; that 15–17-year-olds may or may not be too young, depending on their circumstances; and that 18-year-olds are generally “old enough.” Policies and programs should focus on capacity building and the creation of an enabling environment for making safe and voluntary transitions among all age groups, but particularly among 10–14-year-olds, whose sexual and reproductive health and rights are so clearly at stake.