Birds as a rule can be difficult to judge age (after leaving the nest) since they are missing teeth (having a much lighter and just as efficient beak) as compared to mammals. One way to judge aging in a mammal patient is to see wear (or lack thereof) on the teeth. Mammals and birds have many differences and this lacking teeth is considered one of the ways they are beautifully adapted for being a flighted animal.
One way we can judge bird aging, for at least a short period of a young bird’s life, is to know how certain patterns on the feathering change as the bird develops, and certain physical features.
A couple such changes happen in the Congo African Grey more info
Youngsters have very dark irises, which lighten over time. As the bird matures, it becomes white or yellow (even darkening to a deep yellow color).
Also, the tail feathers are a dull greyish red with grey tips; the tail feathers brighten over time to a bright red.
Our first example is a youngster about 6-7 months of age, and you can see the iris around the normally dark pupil, and the tail feathers.
The second set of pictures is from one 24 years old.