I only have a just over a years experience of meat birds. So I'm no expert on any of it. In my experience, across the 3 breeds I've done. Cobb/ross, hubbard, sasso... I've found the hubbards and sasso rather tough at 16 weeks old. I start dispatching at 6 weeks, sometimes younger. I dispatch whoever looks wobbly on their legs.
We've tried to fattened up other non table birds, those boys we've hatched and don't want to keep. Dispatch them at aged 6 months. And in my opinion it's not worth it, tough as heck.