Usually the breakdown of ingredients, as for instance puppies need more protein than adult dogs. Some are made in smaller kibble, although some companies make large breed puppy and junior food. Some people feed smaller active breeds on puppy food all their lives, as it helps them keep weight on. Look on the bags, they have an analysis of whats in them, or look on the company websites, as most of them have details listed.
easier on the pup's tum after being weaned off mother's milk.
If you put them straight on grown up animal food , like human babies they will be sick as in chuck up and you will probably harm them.
Depends on the type of food. As Lankeela says, there is a different composition of nutrients. Puppy food is often richer than adult food. Large breed puppy food will have lower energy levels so the dogs grow slower.