Teach your toddler the human sign when they are ready to grasp generalities or more abstract concepts. Start with the immediate persons frequently seen by your child: you, your spouse, their siblings and grandparents, cousins, nanny, doctor, and so on. As they see that age and name do not matter, all these persons including themselves are human.
This is a useful flash card taught best in conjunction with the specific persons your child is constantly in contact with. It is also useful to teach this sign when you begin teaching your child the different body parts. Contrast humans with other living things such as animals and plants as well.