The spiritweb is a mysterious concept in the cosmere. To this day, very little is known about it. However, it seems likely that the spiritweb is an individual's spiritual aspect. Evidence suggests that spiritweb are laid out in a specific pattern in a person. This pattern forms a network of lines and points ... or veins.
The actual outlet of the power is not chosen by the practitioner, but instead is hardwritten into their spiritweb.
The spiritweb is related to 'spiritual DNA' - a person's spiritual heritage from living on a specific Shardworld. It is also passed down like Earthly DNA, from parents to children. The spiritweb is also implied to determine which manifestations of Investiture a person can utilize. Some manifestations of Investiture seem to be built into a person's spiritual DNA, like Allomancy and Feruchemy. Other times, such as in Hemalurgy and potentially Surgebinding, involve manipulating spiritual DNA. In Hemalurgy, portions of the spiritweb seem to be ripped off of one person and grafted onto another person's spiritweb. A Surgebinder's Nahel bond with a spren may work in a similar way; a spren becoming spiritually attached to a certain point on a person's spiritweb, sort of like a non-invasive form of Hemalurgy.
|“|| ... There's a perfect version of everyone somewhere. It's a mix of their Connection to places, people, and times with raw Investiture. The soul, you might say.
(Note that over time, a person's perception of themselves shapes their cognitive aspect as well, and the cognitive aspect can interfere with the spiritual aspect trying to make the physical aspect repair itself.) Healing in the cosmere often works by aligning your physical self with your spiritual self - making the physical regrow. More powerful forms of Investiture can repair the soul as well.
However, your age is part of your Connection to places, people, and times. Your soul "knows" things, like where you were born, what Investiture you are aligned with, and - yes - how old you are. When you're healing yourself, you're restoring yourself to a perfect state - when you're done, everything is good. When you're changing your age, however, you are transforming yourself to something unnatural. Against what your soul understands to be true.
So the spiritual aspect will push for a restoration to the way you should be. With this compounding hack, you're not changing Connection; it's a purely Physical Realm change.
This dichotomy cannot remain for long. And the greater the disparity, the more pressure the spirit will exert. Ten or twenty years won't matter much. A thousand will matter a lot. So the only way to use compounding to change your age is to store up all this extra youth in a metalmind, then be constantly tapping it to counteract the soul's attempt to restore you to how you should be.
Yes, all of this means there are FAR more efficient means of counteracting aging than the one used by the Lord Ruler. It's a hack, and not meant to be terribly efficient. Eventually, he wouldn't have been able to maintain himself this way at all. Changing Connection (or even involving one's cognitive aspect a little more) would have been far more efficient, though actively more difficult.
Q&A with BrandonEdit
A. No, because the Nahel bond is already filling in those cracks, so you would have to rip it off to put something else in there.
Q. So the Shard wouldn't be able to heal ... ?
A. Well, the Shard ... Like, here's the thing we have to get at with this, what we're getting at, which is the question of, for instance, is Kaladin's depression a flaw in him that needs to be healed? And that is a question for philosophers. There are certainly people in the cosmere and outside the cosmere that say yes, this needs to be healed, but what about somebody who's ... say, someone who is autistic, and their mind just works in a different way, and this way allows a certain bond to happen that couldn't otherwise happen? Is that a flaw, or - is it a bug or a feature, to speak in coding terms? Is what's up with Kaladin a bug or a feature? I know that my wife would probably get rid of her depression if she could, but it's also fundamental in how she sees the world and who she is, would that change her into a different person? And things like this. So, I want you when you discuss this, to be very careful about treating mental illness as a flaw as opposed to an aspect of a human personality that allows certain different things to happen. Does that make sense?
Q. The way I was sort of thinking, was, could Odium say, "Oh, I'm just going to fix this" and then [ ... ]?
A. Right, right, yeah. If he - if there was - that is possible, but it would be hard to do without the consent of the person, but that is possible. You can fix somebody in a way that they didn’t want to be fixed, and it would ruin things.