Basic data on age-related neuroanatomical changes across the juvenile to adult period in nonhuman primates is sparse, and this gap in knowledge is a serious impediment to translational research aimed at understanding brain development across the lifespan. In this study, magnetic resonance images were analyzed for fifteen mother-reared, socially-housed bonnet macaques (Macaca radiata) in three age groups: juvenile, adolescent, and adult. These data are the first to show age-related changes in gray:white matter ratio and corpus callosum size in bonnet macaques. Juvenile monkeys had higher overall gray:white matter ratio as compared to adolescent and adult monkeys. Corpus callosum (CC) size varied significantly as a function of age and CC region. Total brain volume was significantly lower for juvenile monkeys as compared to both adolescents and adults. These results are consistent in pattern with age-related changes in gray:white matter ratio and regional CC differences observed in humans. Continued study of the animals in this cross-sectional study will provide an important means of determining whether differences observed between age groups reflect developmental differences due to variation in the rate of maturation of CC regions.