was wondering at what age should i harvest them?
What are your concerns? I don't know what criteria you are looking at. Are you worried about how much meat you will get, flavor or tenderness, when they will start mating your pullets or fighting each other, or start crowing? Maybe something else? Why are you asking?
I process dual purpose birds like Australorp. Each cockerel grows at his own rate and reaches his own size. I generally start butchering some at 16 weeks and try to have them all done by 23 weeks. I figure by 16 weeks most have filled out enough and by 23 weeks they've finished any growth spurt. So if the amount of meat you are going to get is a criteria I'd suggest waiting at least until 16 weeks. The more you wait the more meat until around 23 weeks. But each one grows at a different rate and to a different final size.
As cockerels go through puberty the hormones change the flavor and texture of the meat. Some people like that extra flavor, some do not. They go through puberty at different ages. Some 16-week-olds can be fairly strong, some older ones are still pretty mild.
Same thing with texture. You can cook any chicken of any sex and any age and come away with a gourmet meal but you have to cook them the right way for their age and sex. Like flavor, some people have more trouble with texture than others. If you want to grill or fry them, 16 weeks could be too old. Personally I bake mine and they come out great.
Some cockerels hit puberty and start mating pullets or start fighting as young as 12 weeks. Others don't do much even at 23 weeks. As long as none get hurt it doesn't bother me but some people freak out at the thought. I monitor the situation and take action if I need to but most years I don't need to.
Some cockerels start crowing before 12 weeks, others wait a lot longer. If you are concerned about your neighbors hearing them crow then you need to monitor it. Some people say they butcher their cockerels at 12 weeks to try to prevent crowing.
If you can tell us why you want an age maybe we can help you with specifics. Without that information we are just guessing.
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