Just want to post a few pics because of a recent discussion on Prairie Falcon ageing. Sometimes it's not so easy to tell the age of a Prairie Falcon since the juvenile and adult plumages are very similar to each other. Flesh parts (cere, feet, orbital ring) on the larger falcons (Prairie, Peregrine, and Gyrfalcon) turns from pale bluish to bright yellow, but the time it takes for these parts to change color varies quite a bit, and in Gyrfalcon, it takes longer to change than in Prairie and Peregrine. In fact, oddly, many juvenile Peregrines have yellow feet (but often not ceres) by early fall. Also, the flesh parts of males tend to change quicker than on females, same with eye color of accipiters, but of course this varies somewhat individually. By the way, Prairie Falcon gets its adult plumage the first time it molts in its 2nd year.
There are plumage differences between juvenile and adult Prairie Falcons, and often it is easy to see, but sometimes it is extremely confusing. Juveniles typically have short streaks on the underbody, where adults have more rounded spots. The head on adults is more finely streaked than the plainer brown head of juveniles, and the pale spots on the back of the head are less white. The upperwing coverts have broader buffy tips, and the secondary and primary coverts are more heavily barred an adults than on juveniles. The tail of adults usually shows more complete bands looking paler overall than on juveniles.
Here are some composites of juveniles on the left and adults on the right (note how similar the two ages appear), and a head shot of a juvenile in September that is already changing color on the cere ('click' to enlarge).