The different classes of poultry are marketed on the bases of age.
Age influences tenderness and fat content and so indicates the cooking methods that are appropriate.
Signs of youth in poultry are smooth leg skin, supple wing joint, pliable keel or breast bone, and the presence of pin feathers. Young birds have no hairs on the skin and little subcutaneous fat.
Poultry, like meats in general, is tough if it is cooked before or while the meat is still in rigor.
Poultry goes into and out of rigor more rapidly than other meats.