What are the differences between puppy, Adult and Senior?

Put simply, puppy (up to a year old) food is a bit higher in protein, fat and minerals to take account of a puppy's rapid growth and skeletal immaturity. Kibble often has smaller sized pieces for smaller mouths. Food for mature dogs is often lower in fat to compensate for reduced exercise and help guard against obesity. It might also have higher levels of things like chondoitrin and glucosamine to support joints, and lower protein to be easier on the kidneys. There is a lot of debate about the benefit of senior foods against sensible feeding of normal foods with appropriate supplements.

@excuseme is a raw food guru and may have more detail on how that is fed at life stages.

 

I am not very good at this king of question.

I have always fed young and old dogs the same food.
From what I can see with regards the kibbles, there is very little difference, obviously the kibble size. Any differences will be miniscule.
I think the different life stage and breed specific foods are really just a modern day markrting gimmick.

"Millies Wolfheart", (one of the better quality grain free kibbles ) say their kibbles are suitable for all stages from puppies to oldies.

Raw fed pets and wild dogs with "good variety" of product will eat the same diet both young and old. It will just be the amount consumed that will make the difference.

 

Honestly, the main difference seems to be the price? When you read the list of ingredients and the nutritional values on the back the differences seem to be minimal.

 

All the above is correct in the difference between life stage foods. For me a Senior diet simply enforces the condition of getting old and should be avoided. (I can bore you senseless on why this is the case) but there is an old but interesting study presented by Kealy Richard D., Phd; Factors influencing lean body mass in aging dogs. Proceedings of the 1998 Purina Nutrition Forum in which he shows that Senior dog fed on a regular adult diet vs a senior diet live longer. Make of it what you will.