If it were that simple, MX wouldn't of cared about his age.
There is probably some kind of literal death instead of fading into the RoD or Kingdom Hearts.
From what Nomura said in the BbS Ultimania interview, there is
no such thing as death in the sense of a person's "self" ceasing to exist, just "sleep."
But it's pretty clear that "sleep" is normally a permanent state, rather than something people can return from whenever they please. If Master Xehanort's body died before he found a new one, he wouldn't have a chance to come back. The only reason the characters Sora needs to save can be awakened is because their circumstances are anything but normal - Terra, Ven, and Aqua all have existing bodies, Axel's probably just going to come back as Lea, and if Namine is any indication, Roxas and Xion might not necessarily even get new bodies and could have their suffering removed in some other way.
As for Eraqus, I'm not sure he needs/wants/is able to come back. He seems to be mostly protecting Terra at this point, and might just decide to move on once he's sure Terra will be okay.