When I butcher dual-purp birds, any over about 14 weeks tend to be chewy, so I cook them in the crock pot, or pressure can the meat. I have a batch to do soon, I plan to freeze some whole, and the rest, I'll remove and de-bone the breasts and thighs to pressure can. The remainder I'll freeze to cook in the crock pot. I may do that with all of them, if I have time.
That home-canned chicken is yummy, and is handy for quick meals.
It helps to let them rest in the fridge from 48hrs to a week, before you freeze or cook them, but that's not really enough to make a tough bird tender. The crock pot has never failed me, though. If it's not tender in about 6 hours, just wait, It'll get there. I usually cook on "high" in the CP, that's just a simmer.
One that I processed at about 8 months old, was still tough after 5 hrs, and I was ready to go to sleep. I turned the pot down on low and went to bed. Next morning, the aroma was amazing! The bird was tender, the broth was excellent. I took it to a friend's house for dinner that evening. We just served the meat right out of the crock-pot, and ladled out small bowls of the broth for dipping toasted hard rolls in. Along with a salad and some green beans, it was a very satisfying meal! One of my friends took some of the meat and broth to make a soup for lunch the next day. She loves my crock-pot birds!
A 16 week old bird can be slow roasted (250F-300F)in a clay chicken cooker, or in an oven bag, for about 4 hours, and be tender and moist. I let them cook until the meat's coming off the bones. As long as you don't let it dry out, it's great.
They don't make good fryers, but just about any slow, high moisture method works well. They don't have to be immersed in liquid, but they do have to be kept from drying out. Usually the steam in the pot or oven bag is enough to keep them moist, just don't use dry or quick cooking methods. Those are for tender younger birds.