Shelves are divided by life stage, and the divisions are not equally meaningful. Two of the three rest on defined nutrient profiles and one does not.
What growth actually requires
A growing puppy is building tissue as well as maintaining it, which raises requirements for protein, several minerals and total energy relative to body weight.
Calcium and phosphorus are the critical pair. Both the amount and the ratio between them influence skeletal development during the growth period.
Puppies also have limited stomach capacity relative to their needs, which is why growth diets are more energy-dense and fed in more frequent portions.
Why large breeds are separated out
A large-breed puppy grows for longer and adds far more mass, and its skeleton is under load throughout that period.
Excess calcium during growth is a specific concern in these dogs, because their ability to limit absorption of it is not fully developed and skeletal problems are associated with oversupply.
Large-breed growth diets are formulated with controlled calcium and moderate energy density, which is why supplementing them defeats their purpose and is not advisable.
Adult maintenance as the baseline
Once growth finishes, requirements drop to maintaining existing tissue, and energy needs fall sharply relative to the growth period.
The transition point varies with size. Small dogs finish growing considerably earlier than large ones, and switching by age alone can end growth support too soon or continue it too long.
Neutering lowers requirement further, and the combination of that change with a continued growth-diet portion is a common route to weight gain in the first two years.
The senior label is not a defined category
There is no established nutrient profile for old age. Products marketed as senior are formulated to adult maintenance standards with variable adjustments.
Those adjustments differ between manufacturers and sometimes contradict each other, with some reducing protein and others raising it.
The result is that the word carries little consistent information, and two senior foods may differ more from each other than either does from an adult product.
What ageing does change
Older dogs commonly lose muscle mass, and maintaining protein intake matters more rather than less as that happens, provided kidney function is normal.
Activity usually falls while appetite may not, and dental discomfort can alter what a dog is willing to chew.
Because the appropriate adjustment depends on which of these applies to a particular dog, the diet of an older animal is best reviewed with a veterinarian rather than chosen from a label.