This is a very common question and the answer is “whenever you want…and it depends!” Motivated parents can start teaching right out of the womb if they so choose, but generally baby’s do best around 8-11 months of age. You can expect a baby to sign back from 4 months in extreme cases, to 15 months and even longer, however most do well at 8-10 months so that is really the optimal time. No harm is done by starting earlier, but you might get bored or impatient waiting for your baby’s motor coordination to develop allowing them to use their hands to talk. This is the real issue at hand when deciding when to start signing to your baby because ultimately it will be you who gets bored, not your baby.
If you start signing at 4 months will you continue to sign until your baby is 10 months old and finally starts to sign back? Are you the type of parent that gives up easily on things, or are you tenacious and a fighter? Will you want immediate results, or can you keep at it for months with no feedback? Your answers to these questions matter because depending on them, will make your decision for you. There’s no harm in starting to sign early on then giving up only to try later, but signing must be persistent to work properly. I do recommend parents who become stress by this entire affair or who face special and perhaps difficult circumstances do drop signing for a time. Signing should be fun after all and not a chore.
Signing is pretty straightforward, but it’s certainly not easy in the sense of getting instant gratification. The older your baby is when you start to sign with them, the quicker they will sign back to you. If your goal is to get signing the quickest way possible for your particular baby, then the sooner you start, the sooner they will sign back – in relative terms. Starting too early is impossible, your baby won’t give up, but you might!