Which part of a horse can be examined to determine its approximate age?
And the answer: teeth.
Traditionally, the age of a young horse can be determined by which teeth are present and which ones the horse has lost. After that, the age is determined by how much the teeth have worn down, making accurate age estimation relatively easy up until the age of 9 or 10.
Like horses, veterinarians are able to estimate the ages of dogs and cats by looking at their teeth. At one month of age, puppies' milk teeth start pushing through the gums, while permanent canine teeth come in around 5 months. The last permanent teeth to come in are the back molars, which arrive between 5 and 7 months. After that, vets can look to the wear of the teeth to determine age: the smoother the tooth, the older the dog. This same method of study applies to cats, dogs, farm animals, and many other mammals.
While hair, fur, and skin can also be good indicators of the age of an animal, they're not always the most reliable sources of information. Some animals are genetically prone to premature greying, and others can develop vision problems independent of age. Though imperfect and often unspecific, teeth are one of the few consistent and proven methods of determining age in an animal.
Check out this article to learn more about age identification in animals.