FOR MANY DOG BREEDS, THE HARDEST PART — raising the puppy — is over by the time the dog is twelve months old. For the German shepherd puppy owner, however, the difficulties are just beginning at this age. During adolescence, the shepherd has almost reached his full height. He is sexually mature and beginning to fill out. This is also the time period during which a shepherd will challenge his owner, test the boundaries of his home, and need more exercise than ever.