In general, hens are said to be mature when they begin laying eggs. and cocks when they start crowing. This occurs at 16 to 24 weeks.
But these are general terms. Maturity could mean something different when we take into consideration the purpose for which the chickens are kept.
Chickens are mainly kept either for meat or eggs. If chickens are kept for eggs then it is easy to say they are mature when they start laying
Chicken for meat can be said to be mature when they are ready for slaughter. These can actually be pullets and cockerels.
For example, broilers may be slaughtered and eaten at six weeks. Kienyeji chickens take a little bit longer. They are ready for slaughter from 16 weeks and will depend on personal preferences such as weight, si