Look at the wear on their teeth. Once they grow up it is difficult to tell the age of a chicken. Different breeds age at different rates. Individuals also tend to show ages differently. A freshly molted bird looks better than one who hasn't gone through molt.
The only clue I ever got was to look at the legs. A younger one will have more yellowish legs b/c the color (can't remember what chemical it is) gets used in egg production and as they get older - say after a year or so, that yellow color fades to tan or grey or whatever their normal leg color will be.