A chicken is a type of winged domestic bird that cannot fly. With over 150 different breeds, they outnumber every other kind of bird in the world, and have proven to be tremendously useful to humans, primarily as a food source. The average lifespan of a chicken varies, usually between 5 to 7 years -- though there have been cases of chickens that have greatly exceeded this, even beyond 20 years. Although there is no truly reliable way to tell a chicken's exact age, there are some indicators which can give you a rough idea.
Check the chicken's lower legs, called their shanks. Older birds tend to have larger and rougher shanks than younger ones. A younger chicken's shanks are smooth.
Examine the length of the spurs, if they are still present. Smaller spurs will usually indicate a male under 1 year of age, unless they have been removed, in which case they would also be small.
Take note of the size of the eggs, if a female, as well as whether she is laying eggs and how many. Most hens begin laying eggs at between 5 and 6 months of age. The first eggs a chicken lays will be small and soft, increasing over time in size, with production diminishing. A hen will lay more eggs during its first 2 years, tapering off dramatically after about 3 years. Hens that are over 5 years may still lay an egg or two several times a month, but generally their production is significantly limited by that age.
Check for loss of colouring, if a hen. Before beginning to lay eggs, a hen will retain her yellow colour. This colour gradually diminishes, beginning after a couple of weeks, around the vent, eyes and ear lobes. After a couple months, the beak will also lose it's colour, followed by the legs and feet, after about 6 months of egg production. The yellow colour will return at the end of the hen's productive life or after a moult, in the same order in which it disappeared.
Take note of the way the chicken moults. A chicken under a year old will not moult out its chicken feathers.