I'm not sure but people didn't grow reaaallyyyy old before cuz of diseases then again i dont know how far the world is in avatar is to have anti-dotes and all cuz they p. sophisticated.

I think aang and katara were dead :/ and toph daughter was there but for the rest i dont really know, i read it on the wiki that the old characters(main i think) Will prob not appear in that time. just flashbacks stuff.. JUST READ THE AVATAR WIKI OF IT.