Could be one of the ancient weapons. Could be the first ever tree that beared devil fruit. Sort of like in the adam and eve story, this sacred tree grants eternal life but you'll be cursed or something.
You can find this in many cases... Heck, it was in Dragonball Z with the golden apples or something when Goku fell off Snake Way.
I think it's easily believable that the people who seem to be the heads of this world that's filled with magical powers and endless mystery, have found a way to extend their lives. It might not even be an eternal life sort of thing, could be that it's something they need to 'top-up' as it were where they rejuvenate themselves every month or something.
Immortality is that your life doesn't end through time, so living for over 800 years is being pretty immortal

But they can still die, they're not
invincible.
But I can completely see this scene where we have the Gorosei talking how they're going to have to get involved in the fight, then the next chapter we finally see their secret and they rejuvenate back down to a more "prime" state ready for battle. And we're all like damn haha