Between 12 and 15 months, children increase their awareness of themselves as different from other people. Although self-awareness develops more fully later (between 15 and 24 months), sometime during these months children begin to make basic distinctions between themselves and others. For example, when children of this age look in a mirror, they see their own reflections as different from the reflections of others. Many children can use a mirror to find other people who are nearby. Also, children can tell the difference between video images of themselves and video images of other people.
Butterworth, G. (1990a). Self-perception in infancy. In Cicchetti, D., & Beeghly, M. (Eds.). The self in transition. Chicago: University of Chicago Press.