How old should they be? There is no set answer, we all use different ages for a huge variety of reasons.
Some people use behaviors. If they are not allowed to have roosters they may butcher them before or when they start to crow, or maybe they don't want to disturb their neighbors. Many people are really upset when they see how cockerels often behave around pullets or hens and get rid of them then. Some people may butcher as young as 12 weeks for these reasons but there is very little meat on a 12 week old DP cockerel.
How do you feed them? If you are buying practically all they eat you may want to butcher them early, If yours forage for a lot of what they eat feed costs may not be that high.
Cockerels, even the same breed, tend to mature at different ages so this is a bit nebulous, but they go through growth spurts. Most of mine tend to get to a decent size at 16 weeks but I generally wait until 23 weeks. By then practically all of them really slow down growth rate.
How important to you is size? For some people size is the holy grail of butchering cockerels. I like size but there are only two of us. I can get two meals out of a small pullet or hen, a large cockerel mainly means I get leftover chicken for lunch.
To me a huge factor is how you plan to cook it. Their age determines how you can cook them. Generally the older they get the slower you have to cook them and you need a moister method. I hesitate to give specific ages because mine mature at different rates, but as a cockerel goes through puberty the hormones increase the flavor and texture of the meat. Some of us like that change in flavor, some don't and call it "gamey".
You can fry or grill a pretty young bird but at some point they turn into shoe leather if you try that. Your cooking methods need to change to slower and moister. They can be roasted as they age, but eventually that becomes a bit more challenging. I tend to bake my 23-week-olds in a tightly covered pot at 250 F for 3 or so hours. Other methods as they age may be crock pot, pressure cooker, or slow simmer for stew. Coq au Vin, or chicken and dumplings are good for really old birds. You can get a really great tender meal from any age or sex of chicken but you have to choose an appropriate cooking method.
To me those are the main factors that go into when to butcher. If you can tell us which are more important to you we may be able to make specific suggestions. It may take some trial and error for you to find your sweet spot.
Good luck!
