There is no definite way of telling a cat's age accurately. But there are a several factors that you can observe for judging a cat's age. The following link discusses these factors in detail.
www.surroundedbycats.com

 Actually , dogs age 7 years to one human yr and cats age 10yrs to one human yr , the vet should be able to determine the animals age by its teeth

 Basically what you do is how much your cat is in years and months then divide by the amount of people you have living with you its that simple (;