Let me touch on a subject that has been published already and I think is fairly well-known. Juvenile male Kestrels molt their body feathers in their first fall (often on migration). This is unlike other raptors, making them tricky to age in flight. To start, juveniles have whitish chests with black streaks, but can be seen in different stages of body molt until they acquire a complete adult orange breast by late fall. Some birds molt quicker than others, as is true for all raptors. Often, it is possible for bird banders to age male Kestrels during fall migration, but toward the end of fall, it can be difficult. And, of course, and Kestrel molting flight feathers in fall is an adult. Also, bird banders can look to see if fault bars are present in the wings or tail, but this is impossible to see in flight.

Check out these photos below ("click" to enlarge) and see if you can age them...