Vets look at their teeth and the more plaque the older they are but this isn't a for sure way to tell because every dog and breed is different and some people like me have their dogs teeth cleaned every year. Small dogs have worse plaque than larger ones most of the time.
you can watch its teeth, facial expressions, dog hairs. For example, if it only has some while and small milk teeth, it is a puppy below two months. It changes its fore-tooth when 2-4 months old. And then canine teeth when 4-6 months old. When 1 year old, a dog has full and while teeth, its fore-teeth have not become sharp and stick out.
Examine the dog's teeth. Dogs should have all their permanent teeth in by the time they are 7-months old. If his teeth are clean and white, he is likely a 1-year old or younger.Check for yellowing on the dog's back teeth. If this discoloring is present, your dog may be between 1 and 2.Look for tartar build-up and some tooth wear on the dog's teeth. If this is present at a minimal level, your dog is likely 3 to 5.Inspect the dog's teeth for evidence of disease and a more substantial amount of wear and tartar presence. If this is the case, your dog may be anywhere from 5 to 10.Determine if the dog has some teeth missing, heavy tartar build-up and severe wear. If this is the case, your dog is likely over 10 and needs senior dog level veterinary care.Look at the dog's snout. If she is developing white hair around her nose and whiskers, this dog probably is 7-years old or older.Examine your dog's eyes and feel his coat and skin. If the eyes are opaque and his skin is somewhat greasy to the touch, he may be an older dog.Evaluate the dog's behavior. Signs of old age may include avoiding going up stairs, stretching more often and a low level of interest in playing.