You have a dilemma. Size isn’t important to tenderness, age is, but size comes with age. The problem with dual purpose cockerels like you’ll have is that they get tougher before they get very much meat on them. In my opinion breed isn’t important to this but some cockerels put on meat earlier than others, even if they are the same breed or cross with the same parents.
Another complicating factor is that we all have different tastes and different things we are used to. Some people are quite happy to fry or grill a 15 week old cockerel, others finds that meat to be tough as leather. Most people are happy with the tenderness of a 12 week old cockerel but there just isn’t much meat there, mostly bones. Some of that depends on how you feed them. Brining or aging might make a difference.
I know you are looking for a hard and fast age but I can’t give you that, there are just too many variables. In general I suggest you try something and see how you like it. If it doesn’t work out it’s not a failure, it just means you need to try something different.
The older they are the more they need to be cooked slowly and with moisture. There is an exception to this though, a pressure cooker at high heat (but with moisture) will produce fall off the bone tenderness. The general progression is young birds can be cooked practically any way, including fried or grilled. Next comes roasting. Baking, crock pot, stews, or pressure cooking can handle the older birds. Coq au Vin or chicken and dumplings are traditional ways to handle really old roosters.
I would not dream of processing a dual purpose cockerel before 16 weeks and really prefer them around 23 weeks old. I don’t fry, grill or roast. I wait until they have some meat on them.