They certainly might be 1000 y.o. since e.g. Mushin is a Naryu prince, so from an empire which has since become extinct and its relics are excavated all over the game world. It seems that becoming a guardian grants them something like a semi-divinity status and prolongs the existance. Similarly as fusing with a demon seems to make you ageless like in case of Jinsoyun and Yura. What is interesting is how the Ancient Cemetery Divine Mandate is situated time-wise to the Empyrean Citadel one, since in the first we see Jinsoyun as a child and in case of the second she seems to have been living in the court family from a very young age till early teens when this (seemingly 2nd) botched Mandate happened.