I've only rarely butchered any of my birds at 14 weeks, but I like nice big carcasses and rich, flavorful meat. My favorite butchering age for cockerels is right around 20 weeks because by that time they typically have really nice carcasses. One thing to keep in mind is that not only will their bodies be longer than store-bought chickens (as previously mentioned by
@cactusrota) but the meat will be darker and richer as well. The dark meat will nearly be the color of steak many times, and the white breast meat will be closer to the color of dark meat in store bought chickens. And you will actually learn what chicken truly tastes like.
All of my birds are raised on gamebird feed for the first few weeks of their life, and then I switch them to broiler feed until about 8-10 weeks of age, then down to 18% protein grower feed until about 18 weeks of age, at which point they go on layer feed. Even when penned, NNs will be leaner and far less fatty than what you may be expecting no matter what you feed them.
You may eventually find that crossing your NNs with other breeds results in a flavor profile you prefer. Don't be afraid to experiment.