Preprimary education is increasingly recognized as a pivotal instrument for enhancing educational and life outcomes, yet much of the existing research focuses on fairly short-run outcomes and on children in high-income countries \citep{efh11,wws11, hbdt21}. In 2002, Mexico launched an educational reform that mandated three years of preprimary education, prior to primary schooling, gradually phasing-in the requirement between 2004-08. Using nationwide longitudinal administrative educational data, household survey data, and a quasi-experimental regression discontinuity approach, this paper investigates the medium-term impacts (5 years after the reform, during primary education) and longer-term impacts (almost 20 years post-reform) of the 2004-2005 requirement to attend preprimary for at least one year. Results show that this mandate increased preprimary years, delayed primary-school entry, enhanced fifth- and sixth-grade cognitive scores in math and Spanish, improved noncognitive skills, heightened student engagement, and led to greater educational attainment nearly 20 years post-reform.