Generally speaking, chickens are said to be adults when they reach the point of sexual maturity and begin laying eggs. This typically occurs when the chickens are between 16 and 24 weeks of age, depending on breed, but even some individuals within a breed will develop more quickly or slowly than others. The ballpark age for when a young chicken is considered mature is 18 weeks.
The Egg Laying Process
The life cycle of a chicken starts when an egg is formed within the body of a hen. The yolk forms inside the ovary, travels through the oviduct by way of the hen's vagina. The entire development and egg laying process takes 25 to 26 hours per egg. Once an egg has been laid, it will take approximately 21 days after incubation begins to hatch into a fully developed chick. Healthy baby chicks are covered in soft downy feathers; their eyes are open when they hatch.
Chick Growth and Development
Baby chicks shed their down and begin developing actual feathers when they are between 3 and 6 weeks old. They will also begin to develop deep red combs and wattles. Growth can vary depending on breed, genetics and overall health, so talk to your veterinarian if you have any concerns about the way your birds are developing. The average chicken will live between 8 to 10 years.