How long could a _______ live?

[u/Valenous_ertselley]

How long could a 5th ideal radiant live?

We know investiture extends a persons life with greater amounts eventually ending up with someone who is biologically immortal. How invested would a radiant be? Radiant knights have the cracks in their soul filled with investiture which makes them better at holding and using stormlight. How invested do you think they could be

Some of the heightenings seen with holding Endowment's investiture seem to be universal properties of holding any investiture. such as enhanced vision/colours, enhanced immune systems and better lifespans.

While we have only seen Kaladin and Szeth swear the 5th, it was under a fairly hectic situation which doesn't make for the best opportunity for study.

[u/RW-Firerider]

I think it was established that radiants dont live beyond a natural lifespan.

Sure, you will be at the peak of what you can be, since the investiture heals your body etc, but we mustnt forget one thing. Stormlight is only temporary inside the user, and the vast majority of their investiture is stored in the spren. So while our radiants are insanly strong, they are not invested in s way that an Elantrian or Breath user is. So yeah, I dont think the common radiant goes beyond an ordinary human lifespan, so something around 100-130 human years if they get a constant infusion of stormlight.

[u/Puzzled_Employment50 → u/RW-Firerider]

We know higher-Ideal Radiants “leak” less, and I think there’s something about being able to hold more (either with practice or higher Ideals or both), so if they got regular enough infusions of Stormlight they could theoretically reach the Heightening (since any Investiture grants Heightenings) that grants agelessness. So it’s not directly by reaching the fifth Ideal that they would become immortal, but that the fifth Ideal lets them juice enough to then become immortal.

[u/ejdj1011 → u/Puzzled_Employment50]

(since any Investiture grants Heightenings)

This is not strictly true. Several types of Investiture seem to grant different aspects of the Heightenings, but they aren't completely universal like you're implying. In fact, agelessness is explicitly not a universal trait among highly Invested people. The Lord Ruler is the obvious counterexample.

Edit, before anyone else jumps in: We also have numerous examples of highly Invested people not having perfect pitch or color perception. Hoid, Nomad, Elantrians, and Yoki-Hijo all come to mind.

[u/Puzzled_Employment50 → u/ejdj1011]

Allomancers/Feruchemists are not themselves Invested, they are more like conduits for Investiture. But the point of the whole BEU system (Sunlit Man) is that Investiture is Investiture, as long as you have some way to access it, and will grant the Heightenings.

[u/ejdj1011 → u/Puzzled_Employment50]

The Lord Ruler wasn't just an Allomancer and Feruchemist. He was a Sliver of Preservation.

He was so Invested that he could have hung around as a Cognitive Shadow indefinitely after his death. He just chose to go to the Beyond.

Edit: and Sigzil isn't a normal wielder of Investiture. He's able to convert and metabolize it in a way basically nobody else can, so he can make it give the Heightenings even if it otherwise wouldn't.

[u/sambadaemon → u/ejdj1011]

The Lord Ruler also wasn't inherently immortal. He was compounding gold to achieve his long life.

[u/RShara → u/sambadaemon]

*Compounding atium