We start processing at 8 weeks old. I pick the biggest, and any that have pale combs, limps, or obvious issues. I have 5 that are 10 1/2 weeks old and we are letting them go to see how big they will grow. We do restrict feed to 12 hours a day and free range our birds. After 9 weeks old I started seeing a much higher rate of health issues, but no deaths, as I butchered any that weren't active quickly.
I've read that if you want to grow past 10 weeks, you really need to restrict food intake from the beginning.