The best time (for the bird and for the operator) to castrate a chicken varies based on the stature and vibrancy of the chick and the skill level of the operator. 4 to 8 weeks is what I prefer, because in that age range the chicks recover quickly and I rarely have to mark any as slips. A box of chicks (25 - 50) stays with me about a week so I can prep them (3 days) and check for and treat wind puffs before they go home. I only do older birds if owners are willing to accept the risks. I tell them this: I had my tonsils out when I was four. Mom has pictures of me eating with the family (regular food) the next day. If I had the same operation today, I’d recover much slower. Older birds don’t bounce back the way chicks can. Older birds have a lower survival rate. I reduce the risks whenever possible.Just hauled out my book. My edition is from the middle '50's and it was written for the commercial producer. It says to caponize when the birds are two weeks old and the testicles are the size of a grain of wheat. I think this was before the CornishX were developed and caponizing was the way to go to produce a plump meat bird. It also has a section on using hormones to produce a meat bird instead of caponizing. This last was new to me.
Obviously, you are caponizing for different reasons and what is in my book does not really apply to you. Since you are caponizing an older bird the two incisions make a lot of sense. That's how they castrate elephants by the way and for the same reasons. (Sorry, I couldn't resist throwing that piece of useless information in). As for finding someone to teach me how to caponize, I think I am out of luck. I have yet to talk to anyone locally who has even heard of caponizing, let alone done it. How did you learn? Were you lucky enough to find someone to teach you, did you find a video, or did you just have a book in one hand and the chicken in the other and have at it? Anyway, I admire your courage.