Hi Nofuss
The DWP tend to depreciate any evidence that is more than two years old, however, if you can show it's relevance to your problems now then it should still carry weight so if these are diagnosis letters for a life long condition then I would include, but as is the case for anything that you send, explain the relevance.
In my experience of cases, I have helped with - old relevant evidence has still carried weight.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
