It is possible to recognize 3 different ages:
Juvenile specimen: chest, belly and part of the face whitish, dark gray or brown back, gray beak and shield (small size), brown iris, dark gray tarsus and toes, 9th primary with a narrow tip.
1st to 2nd year specimen: feathering similar to adults, but some juvenile feathers can remain retained on the body and wing coverts, with juvenile flight feathers (retained, without moulting), sides of the tarsus greenish gray (turning in most birds to yellow-orange like in the adult from November).
Adult specimen: uniform gray and black slate feathering, white beak and shield, reddish iris, sides of the tarsus with bright yellow, orange or reddish color, 9th primary with a wide tip.
Common Coots undergo a complete post-nuptial moult, usually finished in October. On the other hand, the post-juvenile moult is a partial moult that only changes the body feathers, usually finished in October. In both types of age, there is a pre-nuptial moult that only covers body feathers.