Based on the cosmere chronology we’ve been told about, Some of the worldhoppers must be VERY old.
There is some time-dialation going on. I’ll explain it eventually. We’re almost to the point where I can start talking about that. Suffice it to say that there’s a mix of both actual slowing of the aging process and relative time going on, depending on the individual. Very few are actually immortal, but it depends on which definition of immortal you mean:
Doesn’t age, but can be killed by conventional means. (You’ve seen some of these in the cosmere, but I’ll leave you to discuss who.)
Heals from wounds, but still ages. (Knights Radiant with Stormlight are like this.)
Reborn when killed. (The Heralds.)
Doesn’t age and can heal, but dependent upon magic to stay this way, and so have distinct weakness to be exploited. (The Lord Ruler, among others.)
Hive beings who are constantly losing individual members, but maintaining a persistent personality spread across all of them, immortal in that as long as too much of the hive isn’t wiped out, the personality can persist. (The sleepless.)
Bits of sapient magic, eternal and endless, though the personality can be “destroyed” in specific ways. (Seons. Spren. Nightblood. Cognitive Shadows, like a certain character from Scadrial.)
Shards (Really just a supercharged version of the previous category.)
And then, of course, there’s Hoid. I’m not going to say which category, if any, he’s in.
Some of these blend together–the Heralds, for example, are technically a variety of cognitive shadow. I’m not saying each of these categories above are distinct, intended to be the end-all definitions. They’re off the cuff groupings I made to explain a point: immortality is a theme of the cosmere works. At its core, it’s a series of experiments on what happens when humans are given the power of deity.