not an easy thing to do but staining on teeth can be indicative of advancing years but may also be a sign of what the cat has been fed - younger cats usually have a softer coat and a more slimline shape but again, that's dependent on feed - is the animal speyed/neutered also affects their appearance - in unneutered males they marked their territory when they reach sexual maturity and female cat's begin 'calling 'when they come in season - both very much before their first year. So you see, not an exact science but the general condition of teeth, eyes and coat (assuming the amimal is reasonably well fed) are indicative but it's often a case of 'pick an age - any age!' A vet should be able to make an educated guess based on an examination.