Like all animals, the nutritional needs of dogs and cats vary at different stages of life. Nutrient needs are high early because of the rapid growth that occurs during the first few months of life, whereas senior pets typically have a slower metabolism and are less active. They may need less energy, but often not other nutrient, intake. More detailed information is available for both cat and dog lifestage needs.
Generally speaking, “puppies” and “kittens” are less than 1 year old. Defining the age ranges for adult and elderly pets is more difficult, due to variation by species, and breed (in dogs). For cats, the Feline Advisory Bureau in the United Kingdom classifies cats as “mature or middle aged” at 7-10, “senior” from 11-14, and "geriatric” at 15+ years of age. For dogs, elderly generally means the last 25% of the lifespan. Dr. Kelly Cassidy provides a useful website about dog longevity that shows variations in aging for different sizes and breeds of dogs.