noylj: consider that were some mentions of changes to the magic (e.g. wands being mentioned as invented later than cobra being alive).
Also consider that with the "rise" of the muggle world (and that "rise" was rome btw) the "age of legends" was replaced by the "age of reason" (titles of 2 books covering those periods). It could be rather easily argued that the golden age of wizardry was first before the rise of rome and then later in the dark ages with merlin heralding it.