The answer is very simply explained. The growth plates, the areas of cartilage at the ends of the leg bones are still developing.
They can become damaged very easily causing early arthritis. This damage is NOT gonna be recovered by growing up and can cause deformities for the rest of their life. It is your responsibility as dog owner to make sure your pup keeps a moderate amount of exercise. Just as kids, pups don't realize when they are overdoing it.
Once they are fully grown, they can go out for much longer, but remember this is completely depending on the breed, temperament, health and many other variables. There is never a one size fits all approach.
How should it be done to not harm the pup?
A good rule is a ratio of five minutes exercise per month of age maximum twice a day until the puppy is fully grown. Don't take your puppy for jogging or jumping until he is fully grown. Make also sure they're fully vaccinated before coming in contact with other dogs. In case this is not done yet you can make short 'walks' in the garden or another safe play area.
Never exercise your puppy on a full stomach as this can contribute to GDV (Gastric Dilatation-Volvulus also know as bloating or twisted stomach) wait minimum 2 hours after dinner to go out. This also counts for adult dogs!
Paw Luna🐾
