If you compare the nutrient profiles of puppy foods to the equivalent adult foods you will find, in the main, that the difference is extra protein. this is required because protein is one of the main building blocks of the body, and the adult foods don't have enough of it for the rapid growth that puppies go through.
With 80/20 foods you find that the protein levels are already high enough for all life stages including weaning, puppy and senior (they need more protein too for different reasons) so one food should work for all; after all in the wild they would all eat the same, depending on what prey they had caught that day, they don't hunt special prey for the young or old.
This is where the debates happen: At Eden we believe that the species appropriate diet is 80% meat/fish (10% bone, 5% liver, 5% other offal) with a little fruit and veg, we also add a few beneficial herbs, chelated vitamins and minerals (for easier absorption, to replace what is lost in the 60 second, 90°C steam cooking) joint support pack.
Just as importand, we feel, is what isn't added. No grains (grass seeds that cause many intolerances and allergic reactions), no artificial colouring or preservatives etc.
It's not only grain free, it's also low carb (<25%) as dogs have difficulty digesting lots of starch due to no salivary amylase (this leaves the pancreas to take up the burden of all amylase production, and in some dogs this leads to pancreatitis and even diabetes). The primary metabolic pathway for dogs to convert food to energy is Fats and then proteins, carbs come last, and any digested carbs that aren't burned off are laid down as body fat.
Treats wise, if you use an 80/20 food then ieally you would want treats that are similar or higher in meat content, I make my own in a dehydrator. Raw bones are great for keeping teeth clean
I hope this helps clarify some of the issues, but if you have any specific questions then feel free to ask.