Puppies have baby teeth which fall out and are replaced in a fairly predictable, age-dependent pattern, so very young dogs can typically be aged accurately by examining their teeth. However, factors such as diet, chewing habits and genetics all have such a large influence that it is not unusual to see a young dog with teeth in poor condition or an old dog with relatively healthy teeth.
Middle-aged and senior dogs often have a haze on the lenses of their eyes. It makes the eyes look a little blue or cloudy.
The distribution of a dog’s weight changes with age. Older dogs will often have fat pads in their lower back area. Loss of muscle occurs as dogs get older, too. A prominent spine and a swayback indicate that a dog is more of a senior citizen than a spring chicken.