Rapid development in Latin America has been accompanied by lifestyle shifts, including changes in time use and social environments. Overweight/obesity has also emerged as a public health challenge. We examined whether lifestyle changes and sexual maturity−related indicators (early pubertal development and having a child) predict increases in adiposity among Peruvian youth. Using longitudinal data from Young Lives, we examined changes in adiposity between ages 8 and 15 years old for the younger cohort and ages 15 and 22 years old for the older cohort. Boys and girls in both cohorts demonstrated substantial increases in age-adjusted adiposity measures, but predictors were different for boys versus girls. For boys, increases in time spent in work and domestic chores predicted increases in adiposity body mass index and BMI-for-age Z-score and increases in time spent sleeping were associated with decreases in adiposity (waist circumference and waist-to-height ratio). For girls, sexual maturity−related indicators (early menarche and childbearing) predicted increases in adiposity, regardless of time use. Potential mechanisms for these results may include diet, physical activity, wealth, and urban−rural residence. Time use among youth was associated with diet quality and physical activity, but in different ways for boys versus girls. Strategies for dealing with rising overweight and obesity should incorporate sex-based specificities.