It's important to remember that, although most puppies have lots of energy, their bodies are immature and not ready for exercise that causes sharp or repetitious impact during the first year. Large and giant breeds should be 18 months or older.
The leg bones grow from areas located near their ends. These soft areas of immature bone are called growth plates (also epiphyseal plates or the epiphysis). At about 12 to 16 months, the growth plates “close” as calcium and minerals harden the soft area. When the hardening process is complete, most growth stops and the growth plates are said to be closed. Before they close, the growth plates can be injured or fractured more easily than mature bone. An injury to the growth plate can cause the bone to stop growing or to grow incorrectly.
To protect your pup against damage to the growth plates, postpone high-impact and leg-twisting activities until you're sure the growth plates are closed. Leaping after flying disks or over jumps, jogging (especially on hard surfaces), and similar activities should be avoided until the pup matures. If your puppy is going to mature at 25 pounds or less, she can begin to take part in “grown-up” exercise at 9 months. If she'll mature into a medium to large dog (25 to 95 pounds), wait until she's at least 14 months old. If she's going to be a really big girl of 100 pounds or more, wait until she's at least 18 months old. If you're really impatient, you can have your puppy x-rayed to determine whether the growth plates are open or closed. Whether you do that or not, it's better to err on the side of safety—a few months of patience could make a lifetime of difference for your dog.