A mature chicken, and especially an old one, has considerable connective tissue that is best tenderized with a slow moist-cooking method such as stewing. If you fried the mature fowl, you would have a tough, chewy bird. Since the young chicken has comparatively little connective tissue, slow moist cooking is not necessary. Moreover, this method of cooking would be counterproductive because the lengthy cooking process would over coagulate the protein molecules. Once again, you would end up with a tough, chewy bird.
** Asian Recipes