I don't know much on the subject, I'll tell you what I've read and my opinions from my chickens.At 12 weeks of age in my opinion a chicken stops growing bone, at least I think by that point there bodily fully developed. There combs will get bigger, and they will lay eggs several weeks later, but still there bone size stops growing about that point. Back in the old days, the mane purpose of chicken was a frying bird. Some people didn't consider it "real meat" and never was as popular as grass fed beef. This is a WEIRD society where we eat more chicken than beef! Joel Salatin tells all about how for most of humanity herbavors (cows, sheep, goats,ect) where raised on large amounts on ranches and nomadic herding, omnivores such as chickens where kept on the side, mostly for the eggs. After 12 weeks of growing, the chickens start putting on muscle mass (by the way, this is why the cornish are so popular, they grown on muscle but the bones are smaller). Back in 1925 a 2 and one half pound chicken (live wheight!) at 16 weeks was expected, my hatcher stock barred rocks are about 3 lbs live wheight at that age. So even hatchery stock can be pretty good. Just my 2 cents.