Many infants begin babbling routinely at around 4 months, often entertaining themselves for long periods by producing new sounds (for example, “muh-muh” or “bah-bah”). Although babbling may begin at this age, many normally developing infants begin babbling in later months.
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.