Many pet owners adopt cats and kittens without knowing their exact age since many cats come into shelters and rescues off the streets. Knowing your cat or kitten's age isn’t imperative, but can help you determine the right diet and care they need.

Look at the teeth
Generally, the younger the cat is, the easier it is to determine an accurate age. If your cat already has their adult teeth, determining the age becomes significantly more difficult. For kittens under 6 months old, using their teeth can be a great factor in pinpointing their age. When kittens are between 2-4 weeks old, they receive their baby incisors, and their canines come in between 3-4 weeks old. The premolars start coming in at 4-6 weeks, and by the time the kitten is 8 weeks old, they should have all of their baby (milk) teeth. Starting at about 4 months old, kittens start to lose their baby teeth and the adult teeth start to emerge. By 6 months old kittens have a full mouth of adult teeth.
Determining a cat's age by teeth over 6 months old can be tricky. Generally, if the back teeth have yellow stains, also known as tartar, the cat can be anywhere between 1-2 years old. If all the cat's teeth contain some tartar buildup, then the cat is likely to be between 3-5 years old. If they have a lot of tartar buildup on their teeth and may have some signs of wear, the cat might be between 5-10 years old. Missing teeth can be an indicator that the cat is about 10-15 years old.

Size and shape
In terms of size for kittens, a general rule of thumb is that they gain a pound for every month of age, meaning a 3-month-old kitten is about 3 lbs. However, this is only accurate until about 6 months of age as well. Also, this can only be an age indicator if the kitten is healthy.
If the cat allows it, feel the body and muscles. Younger cats will more likely have tighter skin and have toned muscles. An older cat might have extra, sagging skin.

Eyes
Kittens have very smooth, clear eye lenses, and as they get older, they become more cloudy and jagged. Like humans and dogs, older cats can also develop cataracts. If a cat has cloudy eyes with jagged irises, it is likely the cat is over 10 years old.

Activity
An obvious characteristic that can determine a cat's age is how active they are. Kittens and young cats are much more playful and active than their cat elders. Whether it is arthritis holding them back, or just plain old age, older cats generally lie around more and don't play with toys as much.