This seems to be an end of life confession, perhaps made in writing rather than spoken. He asks for no forgiveness and states that for half a century no mortal has disturbed Fortunato's bones.En Pace Requiescat which means rest in peace.
From this we can determine that Montresor committed the murder at least fifty years earlier. At the time of the murder, Montresor has had time to rise in prominence and then lose the respect of others ("you are admired as I once was"). It would seem that at the very least, Montresor is 75-80 years old, if not more. That makes it seem likely that it is an end of life confession.