How old are they and what breed? Most egg breeds are very good layers until about 2.5 yrs to 3. Some say they otherwise, but they do start declining. It's hard to say when enough is enough. If you have chickens just so you can sell eggs, you def. don't want to keep a bunch of past their prime hens around. Supplemental lighting is an option, to keep them laying but they do need to molt, especially as we get into winter.
I process my old hens. They are not going to be like grocery store chicken, at all. They aren't even going be like 'broilers' that folks raise just for meat. Their energy all went to egg laying, not putting meat on their bones.
Here is what I do with mine. Infact, I just processed 7 old birds yesterday.
I process them, but instead of plucking, I skin them. So much easier. You can leave them whole, or separate at the joints (that's what I do with them). At this point, you can do 1 of several things. 1) you can cook & eat them. 2) freeze them for later cooking. or 3) pressure can them.
Some people just process a bird when they want one to eat. That's too much work, IMO. I process as many as I can at once. Keep 1 out for cooking and then pressure can the rest. I pressure can it so that I can store it on the shelf and not have to worry about the freezing going out, or the power and losing food. That's just my preference.
They will be tougher. When I keep one out to cook and eat, I usually cook it in the crockpot til the meat falls off the bones and make soup or chicken and dumplins. I have marinaded in the fridge overnight and fried it and it was good. If you let it 'rest' a day in the fridge before you cook it, it helps alot for it to 'relax'. Also, when you kill it, don't let it flop around. Hold it still until it stops- my hubby says that letting it flop around makes it tougher, but I don't know if there is a scientific basis for that though. Good luck!