This is an x-ray of a two week old puppy.
Look at how far the bones have to grow before they become a proper bony joint.
This is why you should NEVER let puppies jump, walk up/down stairs, or over exercise. Doing to much activity at a young age will cause serious issues later in life, or even at a young age as hip dysplasia and other orthopedic conditions rising in puppies.
Remember the puppy rule...for every month increase activity by 15 minutes !
For example, an 8 week old puppy only needs 10 minutes of physical activity a day- a 6 month old puppy needs 30 minutes a day of physical activity!
* Physical activity includes - going for a walk, playing fetch, running with other dogs etc.
Enjoy your puppy...but remember, you wouldn't make your 6 month old baby run a mile a day, so don't make your puppy either!!