In episode 4 of The Dark Crystal: Age of Resistance, the Chamberlain calls for the assistance of a previously unshown Skesis named SkekMal (The Hunter) who appears to live out in the woods and from what Rian says in episode 5, has never been to the castle in his whole time guarding it. He is not shown in the first episode of the series where the Skeksis are shown attempting to draw power from the crystal. So how does he survive out in the woods without the crystal's power?
Minor spoilers if you haven't seen the full series and movie.
It appears the direct influence of the crystal is not required for their longevity, as SkekGra the heretic also survived without the crystal. Looking at both the movie and the series, the crystal's power and essence seem to provide physical restoration, rather than just prolonging life as is. A few examples:
- The almost instantaneous healing of the general in the age of resistance.
- The Skeksis getting stronger and having less physical issues when they started consuming essence.
- SkekUng's instant (but short-lived) physical rejuvenation after consuming podling essence in the original movie.
Also, in the series/movie we've seen only seen one Skeksis that died from anything else than physical damage, and that's SkekSo the emperor in the original movie. While that originally seemed like 'old age', the new series has clearly set it up that he is destroying his body while trying to extract and use 'the darkening'. However, they do age, because the new emperor in the movie clearly seeks to recover his youth.
So without more evidence it seems the abuse of the crystal provides them with massive physical restoration, including that which has been damaged by age, but that even without that Skeksis are a very long-lived race. Which makes sense given that they're the evil half of a seemingly immortal being.