This is an X-ray of a 2 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, 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!
The first 6 months of a growing puppies life will determine how well the pups bones and skeletal system develops for the rest of it's life. Overexerting a puppy can cause serious skeletal damage.... Remember the puppy rule for every month increase activity by 5 minuets! For example an 8 week old puppy only needs 10 minutes physical activity a day - a 6 month old only needs 30 minuets a day of physical activity!! *physical activity includes - puppy play, training. Enjoy your new puppy and take exercise slow. Let the puppy determine when he/she is done playing. Keep activity to just light play in the yard, they don't need to go for walks daily their first 6 months.