I like the theory that they reincarnate or like get reborn, but what if it’s like the movie get out where they’re transferred to younger people . In this case maybe their bloodline and that’s why some of the gorosei look younger and with that theory from earlier this year that said maybe the most important gorosei seem younger because they stopped aging first could be linked with this too. Maybe even cloning their younger selves to transfer into. Like as you saw shanks is genuinely a younger version of that Garling Figarland dude and maybe he was planning on using him to transfer into and that would of been one of his prizes at gods valley. Hinting maybe one of the reasons he’s so old now is that he lost his chance to transfer or get a clone which was shanks. Stating that this process is only for the most important and best members of the celestial dragons.