Theory of mind abilities in old adults did not receive attention until Happé, Winner, and Brownell (1998) found that the theory of mind performance improved with advancing age. However, Maylor, Moulson, Muncer, and Taylor (2002) and Sullivan and Ruffman (2004) reported that the old adults performed worse than the young adults on theory of mind stories. We used "strange stories" (Happé et al., 1998;Maylor et al., 2002) and faux pas stories (Stone Baron-Cohen, Calder, Keane, & Young, 2003) separately to examine the theory of mind abilities of an old and a young group with IQ and educational level matched. We found that the performance of the old group was worse than that of the young group on the faux pas stories, especially in the faux pas understanding, but no significant difference existed between the two age groups in the strange stories understanding. Moreover, the performance on the faux pas and strange stories for both old and young adults was separately independent of fluid intelligence, full-scale IQ, verbal IQ, and performance IQ.

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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.