I'm wondering this too, mainly because Buddy actually steals my older dog's food! He seems to prefer it; I'm not sure if he prefers the fact that it isn't his or if he just doesn't like puppy kibble.
Right now we're feeding Science Diet (I know, I know) Adult Small Bites for Hiro, the adult dog*, and the Science Diet Puppy for Buddy. Buddy's three months old.
*isn't it weird how some adult dogs - even big dogs - refuse to eat large kibble? When I was a kid, I had a chow-husky mix and she would not eat the bigger kibble, but her companion dog, who was a much smaller chow mix, would eat anything.