We provide empirical evidence on the impact of raising the minimum age of marriage to 18 years old on child marriage, early motherhood, and school enrollment in Mexico. Using a difference-in-differences model that takes advantage of the staggered adoption of this reform across states, we show that banning child marriage leads to a large and statistically significant reduction in the number of registered child marriages. However, we find no effect on school attendance or early fertility rates. We provide evidence that the mechanism behind these results is the substitution of formal marriage for informal unions. These findings suggest that when informal unions are a viable option for young couples, raising the minimum age of marriage is not enough to prevent early unions and their negative consequences.