How do veterinarians determine the approximate ages of cats and dogs?

Question

My mother recently adopted a stray cat with a lot of injuries and health issues. She thought that he may have been an older cat who had been abandoned by his owners, but when she took him to the vet, she was told that he was a young adult. How exactly do veterinarians determine the approximate ages of animals? After all, it's not as if they have obvious signs of aging (e.g. wrinkles, gray hair) the way humans do.

Answer 1

As the animal ages tartar builds up on the teeth and wear starts to show on their teeth. Characteristically an animal has a certain amount of wear on their teeth for about a 2-3 year range so based on it's overall dental health the vet should be able to tell within a few years of the animals real age, even though each cat, dog, or horse develops a little differently, the vet can say that it may be around 13-14 or 15-17.

The teeth development and wear can tell the age of a lot of mammals including cats dogs horses and humans, but not for some lagomorpha and rodents such as rabbits and hamsters! :)