A quick browse on the Net will result in various answers , no surprise, and the truth is that I do not like to give an exact time frame to any parent. Children are unique in their development, and sign language is no different than rolling over, sitting up, smiling, crawling, and walking. Your baby will sign according to his/her own time frame.
Babies are often ready to sign back when they sit by themselves and can point. Does that mean we should only start signing at that time? Definitely not. We start talking to them well before this point and the same applies to sign language. In fact, babies who are born to families with deaf parents or siblings are exposed to signing at birth . I do not believe in 'too young' or 'too old' -- you are the expert on your child and your instincts will lead the way. Keep in mind, however, that babies are fast learners and may understand what you are signing well before they are able to sign back.