Here is a photo of an adult Ferruginous Hawk - if you look at some of my other photos they are all juveniles. One of the best ways to tell is the difference in plumage on the legs and the tails. This adult actually has rufous colored feathers all the way down to the talons - in a younger bird they are white (this is also referred to as "feathered tarsi," that part of the bird's body that corresponds to our legs and/or shins).
An adult's tail varies from all white to gray & rufous/brown mottling such as this bird.
Finally, the eye is a darker brown compared to the lighter iris color of a juvenile.
This was taken in Wyoming as well, they do not come to eastern Kansas very often!