Common indicators of a cockatiel's age include appearance of the eyes, appearance of the feathers and behavior. While a cockatiel can live to be 15 to 25 years in age, most physical changes occur during the first year or two, making it easier to identify younger birds and more difficult to determine the age of older birds.
According CocktailsPlusParrots, baby cockatiels have large, black eyes and often have shorter tail and crest feathers than slightly older birds. The birds are normally weaned between seven to 10 weeks of age, but they are often not sold until they are two to three months old.
Adolescence occurs around four to six months and begins at the first molt or shedding of feathers. Up to this time, females and males have the exact same appearance. During adolescence, however, males change in color on their cheeks and on the bottom of their flight feathers. Females become less colorful in appearance.
Courting behavior can also be used to determine a cocketiel's age. At approximately six months of age, male birds begin demonstrating courtship behaviors, such as hopping and strutting. Females become broody at around 10 to 18 months of age and may start burrowing or building a nest in which to lay eggs.