I noticed on Laura Erickson's blog a comment in response to post on the eye color of Sharp-shinned Hawk.
One poster said:
I snapped a photo of what I thought was a sharp-shinned hawk in a tree and noticed the eye color is light blue. I hadn't noticed something like that before. Is it unusual?
and Laura responded
Light blue would be extremely unusual.
I've watched a few fledgling Cooper's Hawks and if you get a good view of them the first thing you notice is their eyes.
Cooper's Hawk fledglings have blue-gray eyes that they retain until around September of their first year (for a few months or less). Then the eye color changes slowly to yellow. As the bird makes it to it's second year the eye turns from yellow to red. You'll see this if you can get close to the fledglings which is not that easy but possible in some urban environments when the birds become used to people.
I have a couple of photos of fledglings taken whilst banding them in Seattle, WA that show this feature. The bird above is a Cooper's Hawk under two months old so it had been fledged for perhaps a month or so after hatching. This photo was taken in on July 23rd, 2009.
Sharp-shinned hawks similarly have gray eyes at fledging which slowly change color in a similar sequence eventually ending as orange-red or red in the adult.
Eye color is probably a factor that males and females use to gauge the age of a mate.