Any age. You can start teaching your baby sign language as early as you want to instruct him. They will understand many signs but you may not see them make the signs themselves until they are physically able at around 8 months or later.
At about 6-9 months. Babies can start to learn sign language as soon as they can maintain focus/attention. Seeing sign language after about 6 months, infants can start to learn basic signs and by around 8 months may begin to have sufficient skills to start signing. Most experts suggest starting somewhere in the 6-9 months age range.