How much an older cat needs to eat depends on the pet’s ideal body weight, whether they are overweight or underweight, and on underlying medical conditions. Often, unlike most species, cats require more calories, not less, as they age. The best thing to do is talk with your veterinarian to determine what food, how much to feed, and how often is right for your cat. How much an older cat needs to eat depends on the pet’s ideal body weight, whether they are overweight or underweight, and on underlying medical conditions. Often, unlike most species, cats require more calories, not less, as they age. The best thing to do is talk with your veterinarian to determine what food, how much to feed, and how often is right for your cat.