Just as an aside, I have ALWAYS switched puppies to adult food between 4-6 months old, and can attest that most breeders do, as well. Here's one way to look at it:
Puppy food is an "accelerated growth food." In the wild, dogs nurse until they are around 4 months of age or so, and then they eat the same thing as adults in the pack. There is no "puppy formula", "adult formula", "weight loss formula", or "senior formula". Once they are no longer getting the only "puppy formula" (nursing), which is during the time that they actually do need "accelerated growth", they grow at a slower, normal rate on the same food as adults. It makes perfect sense to put our pets on "adult" food at the same time as they would be in the wild, per se.
Horse folks also understand the benefits of this. A horse kept in a stall and being feed "hot foods' - grain, corn, sweet feed" will grow quickly but is more likely to be less sound than a horse allowed good pasture and kept at a slower growth rate. That horse is far more likely to be sound much longer.