Asker's rating. 1  Much as grey hair is an indication of age, just as with humans, it's not necessarily so. 2  Dogs age differently just like people. 3  Depends entirely on the dog. 4  some dogs do start showing grey hairs at 4-5 years old.. some start greying at a later age. Just like people, individuals can age at different rates. The best way to determine a dog's age is by having the vet examine the teeth, he can give a fair guess at the age based on the amount of wear and tear, and tartar build up..