Below are the most common causes for diarrhea in cats this age. Please click on the links below for more information on these diseases. Should you seek a one on one consultation with a licensed veterinarian, please visit our Ask the Vet page where you can submit a question 7 days a week, 24 hours a day:
Intestinal parasites
Coronavirus
Inflammatory Bowel Disease
Hyperthyroidism
Kidney Failure
Please bear in mind that diarrhea at this age has the potential to be attributable to any number of underlying systemic diseases, not just disease of the gut. Even cancer is possible. As such, a veterinary visit with a full diagnostic work-up is advisable for your pet.