Yes, teeth are a good indicator of age, especially for kittens. Their "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 veterinarian's use. However, with pets' tooth 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.
Teeth are a good indicator of a cat or kitten's approximate age, but they are not foolproof. When I took our Billy, pictured here at homecoming, I provided him with Billy's date of birth, as he had been born in a cat foster home. 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 development of his teeth.
Male cats reach sexual maturity (puberty) around the age of five months. Signs include territorial spraying (marking their territory with a spay of hormone-enhanced urine with a very strong, distinctive odor). Often their testicles become more prominent.
Female cats will have their first estrus cycle (heat) 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, length of daylight and weight of the cat also have some effect on the time of the queen's first estrus cycle.
A cat's fur or hair, called coat, is baby-fine and soft as a kitten. 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 develop patches of white or gray individual hairs, much as humans do.
Healthy 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 insure him optimum 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.