We've found that the process works best when the child is seven years old or older. When kiddos are younger, there's always the possibility that they might accidentally hurt the dog, and while we choose a dog that has an inherently gentle temperament, we want to make sure the dog and child are in a mutually beneficial relationship.
Other than the general guideline that the child should be at least seven years old, we don't have formal age policies for matching a person with the dog because so much varies from person to person. Each person with a disability has their own capabilities (and each family has their own situation, resources, and desires). Whereas one family may be using the dog mainly for the mother to use the dog as a social bridge for the child, tethering, motivation, and teaching empathy, another family might have a teenager who is able to directly use the dog in terms of a dog/handler team (and thus the parents don't interact with the dog much at all). We realize that this answer includes a lot of generalities, but the hope is that it gives you a better idea of how the dog and person (or family) are matched up.