The following diagram has been widely circulated on Facebook and provides an average age -- lower end being small dogs and the upper end large dogs) -- in which the closures occur. I send home detailed exercise guidelines with my puppies.
Injuries to the growth plate may not heal properly or not heal in time for the puppy to grow up straight and strong. Injury to a growth plate can result in a misshapen or shortened limb which, in turn, can create an incorrect angle to a joint which can make the puppy more prone to yet more injuries when he grows up.
Additional reading: "Puppy Fitness that Fits the Puppy - Age Appropriate Guidelines" by Jane Killion http://www.puppyculture.com/appropriate-exercise.html
