I have a 10 yr old, 12 1/2 lb neutered male cat. He's normally an outside cat but has been wanting to spend time with me inside lately. While petting him I notice that I can feel his spine. He's obviously not thin and I can't really feel his ribs. I have other older cats in a similar condition. Is this something that just happens as they age or is something else going on? He still kneads on me but isn't really purring, which concerns me a little bit.
If your cat's spine seems bonier than normal, but his weight is the same, he may be dehydrated. If he is dehydrated, his subcutaneous fat layer is thinner than normal.
The best way to know if there is something wrong with your cat would be to take him to the vet. The vet will do a through examination, and if it's been awhile since your cat has been seen he may recommend bloodwork. Outdoor cats are at risk of a wide variety of diseases and parasites and something may be affecting him.
Indeed, like humans, cats can have fat distribution change, or even lose some weight just due to age. Maybe even more so with cats than with people, because cats tend to get a little hyperthyroidism with age. A cautionary checkup with the vet is in order whenever you notice any change, however, as cats do not tend to show distress, and sometimes the first sign something is wrong is weight loss.