Why is my memory going bad?

I use to have a nearly photographic memory. I could look at/ read something and remember it in minutes. For example I memorized Robert Frost's The Road Not Taken after two or three readings... That was almost 5 years ago. Now I have a lot more trouble remembering things having to do with school, I find myself studying more and more. Could this be a product of age, exhaustion, stress, depression, or even ADD because I often find myself more and more distracted/ detached.