This is the accepted manuscript. It is currently embargoed pending publication. ; Objective- The optimal age at weaning (AAW) from milk onto solid foods in high-income countries is debated. We tested the suggestion that earlier AAW between ages 3-6 months may promote faster growth during infancy. Study Design- AAW between ages 3.00-6.99 months was reported by 571 mothers of term singletons in a prospective birth cohort study conducted in Cambridge, UK. Infant weight and length were measured at birth and at ages 3 months and 12 months. Anthropometric values were transformed into age and sex-adjusted z-scores. Three linear regression models were performed, including adjustment for confounders in a stepwise manner. Measurements at age 3 months, prior to weaning, were used to consider reverse causality. Results- 72.9% of infants were weaned before age 6 months. AAW between ages 3.0-6.99 months was inversely associated with weight and length (but not body mass index) at 12 months (both p?0.01, adjusted for maternal and demographic factors). These associations were attenuated after adjustment for type of milk feeding and weight and length at age 3 months (prior to weaning) and rapid weight gain between 0-3 months predicted subsequent earlier AAW (p=0.01). Our systematic review identified 2 trials, both reporting null effects of AAW on growth, and 15 observational studies; 10 reported an inverse association between AAW and infant growth. However, 4 of the 6 that tested this found evidence for reverse causality. Conclusion- In high-income countries AAW between ages 3-6 months appears to have neutral effects on infant growth. Inverse associations are likely due to reverse causality. ; European Union, World Cancer Research Foundation International, Medical Research Council, Newlife Foundation, NIHR Cambridge Comprehensive Biomedical Research Center, and University of California San Francisco Pathways Explore Grant