It is more difficult to work out a domestic cat’s age from appearance than it is to do the same thing with humans. Normally, most people can work out a human’s age from appearance to within 5-10 years, which as a percentage of their lifespan is about 10%. To hit that target for cats you’d have to be able to work out their age to within plus or minus 2 years and you can’t do that for adult cats. But you can be accurate when working out the age of kittens from their teeth.
Teeth
Veterinarians can use the development of teeth to assess age. The difficulty for non-vets is to get a good look at the teeth! Cats don’t willingly open their mouths and you can’t force the mouth open unless you use a specific technique, which I think most cat owners are unwilling to try. Also, cats are more single-minded with their owner compared to with vets because they’re scared and submissive at the veterinary clinic.
Deciduous (baby) teeth are gradually replaced by permanent (adult) teeth. The incisors erupt at 3-4 months of age. The incisors are the tiny teeth at the front between the long ‘fangs’ (the canines). Incisors are used to nibble stuff and are used to groom fur. You should be able to see the incisors fairly easily.

The canines emerge at 4-6 months of age. The canines are prominent and easy to see. The molars and pre-molars at the back of the mouth also emerge at 4-6 months.
By 7 months of age the adult teeth are fully developed. Armed with the knowledge of this developmental sequence you can work out early age pretty accurately.
Although vets can assess the age of horses for instance by the amount of wear and tear on the molars which grind food, the same cannot be said about cats as their molars sheer flesh like scissors.
Although the state of a domestic cat’s oral health in general is a pretty good measure of the age of the cat.
Unless the caregiver has trained their cat to accept teeth cleaning as a kitten, almost inevitably a cat’s teeth and gums begin to look a bit unhealthy in middle age plus (around 8-10 years).
Oral health
So, if you can get a glimpse at your cat’s teeth and see signs of age such as calculus and tartar (hardened dental plaque) along the border between the teeth and gums at the back of the mouth and possible gum disease, you’ll know your cat is around 8-12 perhaps.

The cat’s mouth will also smell less healthy when they yawn. That’s a little test that might be employed. It is not very scientific but it may give a clue.
Looking their age
Domestic cats also look old when they are old and they look youthful when young. There is a distinct difference. Although the change in facial appearance is clear between sub-adult and adult the differences between the ages of around 3 years old and 10 years old are quite small.

I think a cat who’s 3 years old will hardly change in appearance generally for the next 7 years. Old cats look old with muscle mass reduction, grey muzzle hairs, tired eyes, lower activity levels, long periods of passivity or being plain static. They’ll snooze a lot more. This is about a change in behavior as much as a change in appearance.

Behavior
And on the topic of behavior there are differences of course between a kitten who is full of bounce and fun and the adult cat who still likes to play as play is play-hunting but with less enthusiasm. In this context cats are like humans. To be honest, a kitten looks and behaves like a kitten.
The problem in assessing age comes in those middle, adult years where things change subtly.
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