Hello Likely
Sorry you are confused. I'm assuming that you mean your XRay might be out of date, that you are concerned whether it won't be acceptable because it is not recent ? Or has your consultant not made mention of it in letter about your condition?
I would ask your GP initially whether there is any confusion about your condition; explain what is puzzling you, and ask whether he might also provide you with written evidence of your diagnosis.
So far as I am aware, XRays don't expire. If you have one which shows degeneration of discs then it will still be 'acceptable', but it won't show the stage of degeneration at the present time, which is really what you want when applying for PIP (or anything else).
Hope this has been helpful? If not, I'm sure other members will be able to reassure you or suggest alternative ways of dealing with your problem.