The beginning of understanding other people’s minds is gradual; at 9 months, an infant may exhibit “joint attention,” the ability to look where another person is looking to see what the other person sees. Other skills, like the ability to understand intentions, may be present as early as 18 months of age. However, two- to three-year-olds still live very much “in the moment,” and it is developmentally normal for them to assume that other people know the same things that they know and to remain very focused on their own feelings, goals, and ideas. Things like waiting, sharing, and hearing the word “no” can be very difficult and require help from a supportive adult.
Meltzoff, A. (1995). Understanding the intentions of others: Re-enactment of intended acts by 18-month-old children. Developmental Psychology, 31, 838
Sabbagh, M. A., & Moses L. J. (2006). Executive functioning and preschoolers