Two experiments were conducted to compare the false belief understanding of children who have no siblings, but have classmates of different ages in kindergarten. In Experiment 1, 4- and 5-year-olds completed two unexpected location tasks. We found that 4-year-olds with classmates of different ages performed significantly better than those with classmates of the same age. This result was replicated in a larger sample in Experiment 2 in which the children were asked to complete an unexpected location task and an unexpected content task. The findings suggested that the presence of minds with varied ages stimulates the social cognitive understanding of young children, particularly for 4-year-olds. The findings of the present study give a particularly clear view of the effect of classmates of different ages on young children's theory of mind development, extending findings in other research on the advantage of having siblings. (Contains 2 tables and 1 figure.)