JG are slower to mature. I’ve had done black ones not crow until closer to 6 months.
One year we butchered 2 cockerels we didn’t need. I think 6-8 months old, both were same age, just can’t remember exactly how old. They were different breeds. The BJG testicles were still very small and un-developed. He really wasn’t crowing, but he sure did look good for a male JG at that age. The other had very large testicles - no surprise as he was crowing lots, mating the girls and being a male. My males begin to crow and take over the flock closer to 8 months or slightly older.