Hens can reach sexual maturity at 5-6 months, but it takes up to 18 months before they are physically and emotionally mature. They should not be bred as soon as they are able to, as they do not have sufficient vitamin, mineral, and calcium stores in their bodies for the demands placed upon them to create eggs, and their bones are still soft and not always able to withstand the rigors of egg production. Emotionally they often do not know what to do with the chicks, or even how to properly incubate the eggs at 6 months of age.
Cockbirds reach sexual maturity a little later, maybe 7-9 months, but like the hens are not completely mature until much older. The males also should not breed just as soon as they are physically able, a little waiting until they are more emotionally mature is suggeste.