I agree with Kathy. Some lines run lean. I have a line that does run lean, and he is still on puppy food. He is also very picky, so it's always a challenge to keep him eating. But he is on puppy food at over a year. His mother only filled out a bit more after she had the pups last year. Her sister, that has not had a litter, is still very skinny and so are the other boys.
My other line, usually around four to six months I change them over. The quality of the food, is, IMO, generally what I look at. A good quality food is just as good as puppy food, without the extra fat in it. The food they make now isn't like the food of 20 years ago.
It is best to ask the breeder, simply because they know the line and how they grow.