12 weeks is just fine. So is 10, or 8. Or maybe 14.
Preparation methods matter. So does the activity levels of the birds. But you should have very tender chicken to at least 12 weeks, likely 14. That's the period I try to take young males, but I have "dual purpose" mutts from hatchery stock who free range, so they need that long to put on some weight, and and also to assist me in sexing them, since I can't rely on pattern indicators as I might with some breeds.
The older the bird, the more flavor, and of course, the more chew. Dry high heat methods are best for young birds, as they age, you want to transition to slower, lower heat, wetter methods (stews, stocks, braises, etc) or mechanical maceration (sausage).