My ISA brown roosters last summer were around 14-16 weeks when I got them butchered, they could still be fried but the dark meat was noticeably darker than store chicken, the dark meat was starting to toughen, it was not what I'd call tough but it was chewier, it could still be eaten fried, I wouldn't want to go much older on a chicken I wanted to fry or grill. The dual purpose breeds will still put on a little size after that but I'd rather sacrifice a little carcass weight to still have birds I can cook any way I like rather than try to squeeze a extra half a pound out of them and have something that's only good for soup or a slow cooker, I prefer to grill or fry most my chicken, I'll roast one occasionally and the older birds will turn out good on a smoker too since that is a lower temp slower cook but I haven't done a very old bird that way, the one I did was around 7 months