Here are a few things veterinarians check to determine the age of a cat:
TeethYes, teeth are a good indicator of age, especially for kittens. Their initial "baby teeth" first emerge between two to four weeks. Their permanent teeth are developing above them, and by three to four months, will start to displace the baby teeth (also called "deciduous teeth.")
In older cats, the amount of staining (tartar) is also an indicator of age which veterinarians use. However, with pets' teeth cleaning products readily available, tartar may not be a good indicator, depending on the diligence of the cat's caregiver in providing a dental care program.
While teeth are a good indicator of a cat or kitten's approximate age, they are not foolproof. As an example, when I took our Billy to our veterinarian for his first visit, I provided him with Billy's date of birth, as he had been born in a cat foster home, and I had been given his birth date when I adopted him from the foster caregiver. Although Billy was then five months old, the vet estimated his biological age to be closer to two months, based on his low body weight (2.5 pounds) and the delayed development of his teeth.
Sexual MaturityMale cats reach sexual maturity (puberty) around the age of five months. Signs include territorial spraying of urine. Often their testicles become more prominent at that time. Many veterinarians now practice early spay and neuter. This not only helps prevent early pregnancies, which are harder both on the mothers and on the kittens, but the surgery itself is said to be easier on the cats at a younger age.
Female cats will have their first heat (estrus cycle) sometime between the ages of five and twelve months. A female cat will tell you she is experiencing estrus very visibly and vocally. According to PetEducation.com, the length of daylight and weight of the cat also have some effect on the time of the queen's first estrus cycle. In case you haven't already figured it out, the female cat can become impregnated and give birth to her first litter by the time she is 7 1/2 months old.
Coat DevelopmentA kitten's fur or hair, called coat, is baby-fine and soft. As a cat ages, his coat will thicken and coarsen somewhat. It may also change color, becoming darker or lighter in shade. When a cat attains senior status, he may even develop patches of white or gray individual hairs, much as humans do upon aging.
EyesHealthy Kittens and cats during their maintenance years have eyes that are very clear and bright, with no evidence of tearing or discharging. Cats in their later years may develop a cloudy appearance of their eyes, including, tearing, and/or discharge.
As I mentioned in my caveat above, these are all very general answers. Your veterinarian is your best partner, not only for determining the current age of your cat but also in planning a care program to ensure him the optimal quality of life and longevity. Veterinarians practice treating the whole cat, rather than treating a symptom or disease, and this philosophy carries over into determining a cat's age. A thorough veterinary examination of the cat's whole body will generally reveal a close approximate age of the cat.