Cornishx birds are fragile, so it's best to do them spring or fall, not in summer's heat. They grow insanely fast, and after about three weeks of age, should have food maybe eight hours a day, not all the time. No roosts, good air flow, we've raised them in a hoop house on grass, moving the hoop house often enough that they have fresh ground.
If you are going to use a local processor, make that appointment BEFORE getting the chicks, so you can time things, because eight or maybe ten weeks is as late as you want to go with them. Two years ago we didn't do that, and managed to keep our little group alive to fourteen weeks. They were BIG, and we were very lucky to not loose any.
The Freedom Rangers from Pennsylvania are more like normal chickens, can free range, and generally act as though they feel well. They take longer to get to size, more like twelve weeks or so, and because they are slightly older, have more flavor when cooked. It costs more to raise them; the Cornishx birds are the most economical, poor things.
Mary