Well High Res noted that most of his stuff hasn't really aged all that well in comparison to the other classics that (with the aside from Black on Both Sides and GKMC which are both fantastic rap albums but usually don't top "greatest rap albums of all time" lists). If you look at many "greatest rap albums of all time lists", both professionally and on personal websites like RYM, albums like Illmatic, Ready To Die, and 36 Chambers are in the Top ten of those lists always, while Eminem's material is much lower on all of those lists. I'm willing to bet that Eminem's early albums would be far higher if it was ten years ago or so.
Not a knock on any of those albums, but it does show that many albums will eventually fade away from time and not be discovered by newer rap fans (Which, IMO, is sometimes a good metric to determine if an album has aged well or aged poorly). I doubt, however, that Illmatic and the like will ever lose interest from newer generations of listeners, kind of like how Led Zeppelin and Dark Side of the Moon keeps on gaining new fans years after its release.