Baby sign language is the use of sign language in order to communicate with infants and toddlers. While infants and toddlers have a desire to communicate their needs and wishes, they lack the ability to do so clearly because the production of speech lags behind cognitive ability in the first years of life.
According to Nancy Cadjan, President of signbabies.com, a line of books, seminars, DVDs, games, and flash cards that advocate baby sign language, babies are developmentally ready for their parents to start signing to them after 4 months old, but won’t be able to sign back until 7-9 months, when they have better coordination.
New York Baby Mind 