Nothing in Hyrule has indefinite time in their body unless through special magic i.e. Ganondorf. Rauru may already be dead but the place where dead spirits go is the Sacred Realm. Saria doesn't physically age but she could die of old age. Darunia, Ruto, Impa and Nabooru an physically age and could just as well die of natural causes and they go to the Sacred Realm and/or inhabit other bodies unless they are resurrected.
It depends. They do reappear in Twilight Princess. And Plus, Ruto, (Or the sage of water) Is dead. But that's because stupid Ganondorf killed him/her/it. And to top it off, he ran off with the sword of sages. (The sword they were trying to kill him with) :(