Many of us in the petfood industry are focused on making products that try to improve lifespan as well as quality of life. The only thing that's been definitively shown to do that from a nutritional as well as medical perspective is caloric restriction.
A very impressive study, conducted over 14 years, looked at Labrador Retriever littermates randomly separated into two groups that received the same living conditions, care, exercise level and food (Lawler et al., 2008). With the experimental group, the amount of calories they consumed was restricted by 25% to ensure the dogs remained lean compared with the control group, which was allowed to become about 10 to 20% overweight.
The dogs that were kept lean lived almost two years longer than the control group. Equally profound is that clinical signs of some of the chronic diseases that we see in older pets, such as osteoarthritis, were much delayed.
This has not been studied in cats, but it's been true in all species studied so far, including monkeys, worms and rats.