Are there any other bird species you're thinking about? Have you had birds before, having an Amazon could be really tricky and well, not easy for a "beginner". Amazons, from what I've heard are: unpredictable, loud, destructive, and very stubborn. There might be other Amazons that might be a little easier for you.
I would have to go along with saying the bird is 7. I'd ask the owner where the bird was purchased and if he's in a vet record to see if the age is there.
RavensGryf is right, the only way to determine a birds health is through an Avian Vet. It also depends if the bird was living indoors or not. If he/she was an outdoor bird there's a higher risk of diseases.
Also, make sure you get to meet the bird first. You might end up with a bird that is aggressive or not the kind of bird you were hoping for. If they refuse to let you meet the bird, don't go for it! Did the person give you the reasons why they need to rehome their Amazon?
Oo! Michelle, what kind of birds are you debating about atm? (Sorry, hope I'm not being too nosey).