C.S. Lewis used to refer to this kind of position as ‘chronological snobbery, a term that osgood410 mentions as well.! At its worst, in the context of moral or political debate, it amounts pretty much to the claim that moral and political claims are, as it were, a matter of fashion. ‘That’s so last year, darling!’ Of course, the only important issue in evaluating an idea is its truth, not its originator, not the date of its origination. This fallacy belongs in the category of irrelevance.