You can use this website to see trends over time and look at how much things historically sell forIf:
https://www.popsike.com/php/quicksearch.php?searchtext=iron+maiden+the+x+factor&sortord=
Age of the item depends less than demand/availability. Any of those early CD era Iron Maiden vinyl albums are going to hold their value because somebody always wants them and there aren't many (not to mention very low demand at the time). If suddenly a thousand sealed X Factors from 1995 went on discogs, the price would drop drastically as there are now plenty of them available.
Someone else would have to check on this, but I also wouldn't be surprised if the prices on the original vinyl copies came down a bit on the secondhand market when all of the albums got reissued. Originals will always be the more coveted versions, but I'm sure interest declined a bit when new versions became available, at least at first.