Look how far the bones have to grow before they become a proper joint. This is why you should never let puppies grow to quickly by feeding "puppy" food, jumping, running up and down stairs, over exercise,get to fat or over train. Doing to much  activity at a young age will cause serious issues in later life, or even at a young age as hip,elbow dysplasia and other orthopaedic conditions are increasing in puppies ! Remember the puppy rule for every month increase activity by 5 minute For example an 8 week old puppy only needs 10 minutes physical activity a day - a 6 month old only needs 30 minutes a day of physical activity eg.playing off lead in a dog park ! *

Physical activity includes - going for a walk, training, playing fetch, playing with other dogs etc. Enjoy your new puppy but remember you would not make a 6 month old baby run or walk a mile a day so don't make your puppy either!