There are a multitude of underlying diseases or medical conditions cats are susceptible/prone to getting in their senior years. If you're concerned about weight loss in an older cat with a normal appetite, it could be a metabolic problem. In either case, bloodwork will be the best shot at giving you an answer.

Pets are SO GOOD at hiding illnesses until they can't anymore. In most cases, by the time you see clinical signs, the disease process has already been going on for awhile.