Although babies are capable of imitating some facial expressions at birth, they become increasingly able to copy adult actions and gestures as they get older. Between 8 and 12 months, babies will imitate a variety of adult (or older sibling) behaviors during play.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.