For me, I like to process about the same time as everyone else who's commented. My personal best indicator, is right after/ during when their colorful and tail feathers are coming in.
It's a catch 22 though, the longer you wait, the more muscle they have... but the quicker you kill - the more tender they are, and the less they eat..
I have a roo, that's about 17 weeks old, his tail feathers have been swooping for about 3 weeks now, I have three more younger birds, by about a month.. that are just getting their colors in... I have them prior out on the range with the laying hens, but caught them last night to put onto feed for 2 weeks- until the day comes... I feel cornfed chicken is much better, quite the same as cornfed steak.
My 17 week old I caught crowing this morning for the first time... It's my thought that when they are crowing, they are maturing, and when that's happening, they're getting tough...
I'm way in on my logic of the tail feathers ordeal...