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, over exercise or over train. Doing to much impact activity at a young age will cause serious issues later in life, or even at a young age as hip dysplasia and other orthopaedic conditions are rising in puppies!
Remember the puppy rule for every month increase activity by 5 minuets! For example an 8 week old puppy only needs 10 minuets physical activity a day - a 6 month old only needs 30 minuets a day of physical activity!!
*physical activity includes - going for a walk, training, playing fetch, running, playing with other dogs etc.
Enjoy your new puppy but remember you wouldn't make a 6 month old baby run a mile a day so don't make your puppy either!
--Lauren Murray, United Kingdom