I didn't notice it – maybe it's just a glitch, but it seems that dragons age creatures of level 7 more often, and the Archangel has been aged the most throughout the game.
Dragons aren't that precious, so I often attack Archangels with them – just in case they get aged? And then it's game over! Well, what else? Then Dreadnoughts, Vampires, and a couple of thousand skeletons join the battle. I don't mind losing skeletons; after such a battle with humans, I'll raise even more. In general, defeating humans with Necropolis is not a problem; let's discuss how to defeat Necropolis with humans.