So White Sand [then Elantris] is earlier... Then how the heck old is Khriss then? Will we ever get an answer as to why every worldhopper is flippin' immortal?
There is some time-dilation 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.
Implying that some are actually immortal? :D
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--which, at their core, are experiments on what happens when men are given the power of deity.
Heals from wounds, but still ages.
Would Bloodmaker Ferrings exist in this category as well? If not, what about someone Compounding Gold?
Yes, you are correct.
As a Bloodmaker ages, what keeps them from healing the damage and carrying on as a very old, but very healthy person? Do they come to a point where they can't store enough health to stave off the aches, pains, diseases, and other things that come with old age?
This makes sense for traditional Feruchemy as it is end-neutral, so storing health becomes a zero sum game - eventually, you're going to get sick and you're not going to be able to overcome it with your natural healing ability no matter how much you manipulate it with a goldmind.
...Unless you've got a supply of Identity-less goldminds lying around. Would a Bloodmaker with a sufficient source of Identity-less goldminds (or the ability to compound, thus bypassing the end-neutral part of Feruchemy) eventually just die from being too old?
Basically, yes. They can heal their body to match their spiritual ideal, but some things (like some genetic diseases, and age-related illnesses) are seen as part of the ideal. Depends on several factors.