You may be surprised. While you may not see anything like in the photo, a very high percentage of older pets have very poor oral health. Not only will infected gums and loose, plaque-encrusted teeth be painful for the pet, but also an infected mouth can seed bacteria into the blood stream to be carried throughout the pet's body. Cardiac and renal pathology are often a direct result of blood-borne bacteria whose origin is the mouth. Many older dogs and cats respond very positively after having had a dentistry with tooth extractions.
Antibiotics are a requirement after any dental procedures in an infected mouth environment.
As for the rest of the digestive tract there is often a gradual decline in overall function. For example the movement of food along the tract may be slowed which can result in constipation. Digestive enzymes often are not secreted in proper amounts to fully digest the food. This results in loose stools and a decrease in nutrient uptake. Take a look at the photo on the right and you will see an X-ray of a dog that is constipated.