We investigate the impact of school starting age on teenage marriage and motherhood in Vietnam, where the rates of both are rising rapidly. We exploit a discontinuity in the age at which children start school and use regression discontinuity methods to identify the causal effect of school starting age on premature marriage—a first in the literature—and early motherhood. We find that girls who start school earlier and who are therefore younger relative to their classmates are significantly more likely to marry and/or give birth in their teenage years. We argue that the negative effects of starting school early are transmitted through adverse peer influences. We also determine that school starting age impacts are heterogeneous across girl subgroups. The significant effects of school starting age are concentrated among teenage girls who are members of ethnic minorities, whose mothers have relatively less education, whose households are relatively poor, and/or who live in rural areas. Girls that fall into these subgroups are more likely to benefit from starting school later. Finally, we present some preliminary and suggestive evidence that participation in extracurricular activities may help mitigate negative school-based peer effects.