I raise and periodically process cockerels from several breeds of dual-purpose chickens, and have also raised Black Australorps (and crosses) in the past. I find that waiting until they're 5 months old results in a larger carcass - if space, temperament, weather, and other factors permit. Sometimes, they don't.
As has already been advised, the dressed-out bird should rest for several days after processing. My cooking method of choice is the electric pressure cooker (EPC); with enough time (regardless, it will be much quicker than a slow cooker), even older birds are rendered tender. Once the bird is cooked and de-boned, I make the most amazing bone broth in the EPC, too. Highly recommend.
As has already been advised, the dressed-out bird should rest for several days after processing. My cooking method of choice is the electric pressure cooker (EPC); with enough time (regardless, it will be much quicker than a slow cooker), even older birds are rendered tender. Once the bird is cooked and de-boned, I make the most amazing bone broth in the EPC, too. Highly recommend.