It's not uncommon for someone to adopt a cat or find a stray cat and not be sure how old he is. Once cats are adults, they can be very difficult to "age". Some factors that can help include the following:
Tooth development – Cats get their adult teeth at approximate ages and therefore can be used to help "age" a cat. However, by six months of age, cats have all their adult teeth making it very difficult to "age" a mature cat.
Tartar development – The condition of the teeth can be helpful as they will often develop tartar and calculus with time. However the rate of tartar development varies from cat to cat making it difficult to use.
Size – Evaluation of a cat's size to determine if they are full grown can help "age" a cat. This may be difficult in some cats, as you don't know what their true adult size is supposed to be.
Attitude – In some cases, this can be somewhat helpful such as if they act like a kitten. However, this is not very reliable as some cats will be more playful than others.
Grey Hair – Some cats will have graying of hair on the face, especially around the muzzle. It is not reliable as cats can gray at different ages, just like people.
Eye changes – As a cats ages, the lens of the eye become more dense and makes the pupil look "cloudy". This change is normal aging change called lenticular sclerosis. This change occurs most often around 5 – 6 years of age.
All of these factors have limitations. Many times, aging an adult cat will be based on a combination of the all of the above factors combined with experience of seeing lots of cats of which you know the ages.
If you what to know your cats age, ask someone with lots of animal experience such as your veterinarian.