OBJECTIVE: We examined the longitudinal associations of age at achieving gross motor milestones and children’s development in a US cohort of singletons and twins. METHODS: In the Upstate KIDS study, a population-based study of children born between 2008 and 2010, information on age at achievement of motor milestones and developmental skills was available in 599 children (314 singletons, 259 twins, and 26 triplets). Mothers reported their children’s major motor milestones at ∼4, 8, 12, 18, and 24 months. At age 4 years, children’s development was clinically assessed by using the Battelle Developmental Inventory, Second Edition (BDI-2). Primary analyses by using multivariate linear regressions were conducted in singletons. We also examined the associations in twins. RESULTS: Later achievement of standing with assistance predicted lower BDI-2 scores in singletons in adjusted models (B per SD of age at achievement, –21.9 [95% confidence interval (CI), –41.5 to –2.2]). Post hoc analysis on age of standing with assistance showed that associations were driven by differences in adaptive skills (B = –5.3 [95% CI, –9.0 to –1.6]) and cognitive skills (B = –5.9 [95% CI, –11.5 to –0.4]). Analyses restricted to twins suggested no association between the age at achievement of milestones and total BDI-2 score after adjustment for gestational age and birth weight. CONCLUSIONS: This study provides evidence that the age of achieving motor milestones may be an important basis for various aspects of later child development. In twins, key predictors of later development (eg, perinatal factors) overshadow the predictive role of milestones in infancy.