When children develop the ability to deceive others is somewhat controversial. Although some researchers believe children can be deceptive by age 3, most believe that deliberate “lying” does not emerge until children are 4 to 5 years old. To lie, a child must attempt to mislead another into believing something false. But several experiments have shown that children under age 4 don’t realize that people can have false or wrong beliefs about something. Between ages 4 and 5, children begin to understand that people can hold false beliefs about things in the world. It’s around this time that children become more aware that they can also be deceived.
Peskin, as cited by Perner, J. (1991). Understanding the representational mind. Cambridge, MA: MIT Press.