What tells you the age from the back of this bird? Lack of any pattern to the feathers? Tail feathers? When I see these, after they've fledged, they do have white fringes to the feathers and some white patches. Also they have pinkish breast feathers.
So is it the lack of any white in the feathers, the overall greyness, or something else that you are using to age this bird? Bear in mind that my eyesight isn't great, so you might see something that I can't|=)|