Will Falconer, DVM Moderator
I think it's most often appropriate to give one rabies vaccination in a dog over 4 months of age. The likelihood is lifetime immunity after this age. But, and this is a big "but:" do this only when the dog is in robust health, showing zero symptoms of illness.
How to get there? Diet, avoidance of toxins, but most often with a vet homeopath's help. Diet and healthy living can help a lot, but it's likely not enough to root out chronic disease.