15 weeks is starting to get on past the age you are shooting for to have a tender chicken for frying. If they got tough on you you should have to adjust your cooking method, a slower roast or crock pot should tenderize them quite well. Supermarket chicken is processed by 8 weeks if not before, that is why it's tender, I would suggest going with a meat bird if you want grilling frying birds, something that will be ready to butcher by 8-12 weeks ideally, the meat birds will have a lot more meat as well