Often I'm asked "What is the optimal age to introduce signs to our baby?". I tell families I started signing with Joshua and Sabrina when they were 6 months...but now, knowing what I know, I would start much earlier...from birth.
Babies have the motor skills to sign back between 8 and 11 months, typically when first waves for "BYE-BYE" and the "PICK ME UP" gesture starts. So this is when productions or expression of signs can typically appear. But understanding or the ability to receive signs can occur much earlier.
Babies younger than 8 months may get excited about seeing the sign for "BATH" or "MILK". Often at trade shows and presentations, mom ask me not to sign "MILK" in my examples because their young babies understand the sign!
Joanne, WeeHands Instructor in Whitby, tells a story about her son, Richard, who disliked being laid down...but when Joanne would lay him down and sign "DIAPER CHANGE" he was okay with being laid down. Maybe he was understanding, "Ok, this lay down means "DIAPER CHANGE" not sleep time!"
I did get an email a few years ago from one mom I taught. Her daughter at 5 months was signing an approximation (B-shape hands, not S-shape or X-shape hands) of the sign for "CHANGE", after her diaper was dirty!
The area of a baby's brain that is responsible for understanding what is seen (occipital lobe) develops before the area of what is heard (temporal lobe). So young babies can understand what they see before they understand what they hear!
Sign early, sign often and make it fun & relevant for baby!