What is a good minimum weight / age I can aim for so I'm prepared to do the deed at the right time (allowing that all birds are individuals and won't necessarily mature at the same rate)?
I'll assume they are dual purpose breeds. I don't go by weight because they can be so different, even if they are full brothers and the same breed. Age is a better guide but you can still have big differences between individuals. I'll talk about average ages.
Once they hit puberty and the hormones start to flow the boy's meat gains texture and flavor. The texture changes can dictate how to best cook them. Some people like that change in flavor, some consider it gamey and hate it. We are individuals too and have our own prefernces.
What are your goals with the meat. Some people butcher at 12 weeks to make sure they don't crow or so the boys don't start harassing the girls when puberty hits. There is very little meat there but it is tender enough to be fried or grilled. Not much flavor either, a lot like store chicken.
For many people 16 weeks is a good time. This is when they typically have a reasonable amount of meat and the flavor isn't all that strong. For most people used to the Cornish X from the store that is butchered at 6 to 8 weeks of age 16 weeks is a bit late for frying and grilling, you may need to consider an alternate cooking method.
Personally I like to butcher at 23 weeks. Generally they have hit the end of rapid growth. The flavor increase is noticeable but not overpowering to me. They will continue to gain weight but at a slower pace. I like to bake these.
I don't know how many you have this year or what you may have in the future. And I do not know your experience level butchering. I generally suggest you experiment, butcher at different ages so you gain experience in how they suit your purposes at a certain age. Find the sweet spot for you that comes closest to meeting your preferences under your feeding and management methods for your breeds.
