In The Dark Crystal we see that the Skeksis drink the vital essence of other creatures to retain their youth. Or more specifically only the Emperor of the Skeksis is allowed to drink the vital essence. But when we meet the Emperor, he's dying. He's frail and falling apart. He dies by literally disintegrating into dust.
So if he is drinking the vital essence, then why is he so much older and weak than the other Skeksis. My theory is that while the vital essence gives temporary youth, the body rebounds hard after its effects wear off.
We see the General drink some of the life essence of a Podling, and he doesn't stay young for very long (half a minute at most). We see him go right back to being old.
As well, we are told by the Scientist says that it worked better with Gelflings. So my assumption is that as long as they could continuously consume vital essence, the emperor stayed young and strong. When they could only rely on the lesser vital essence of Podlings the rebound effects were significant. This caused the Emperor to become the broken, shell which we see in the movie.
There is also some evidence in some of the expanded works to suggest this as well. In the Creation Myths comics, the Skeksis are shown as using all four hands like the Mystics. When Gelflings were in abundance all of the Skeksis partook in the drinking of vital essence. But when they became more scarce only the Emperor could. We see in the movies that the Skeksis still have four arms (it can be seen when the Chamberlain gets disrobed). However, their second set of arms have atrophied to the point where they are shriveled up and unusable. I believe that this deterioration may have been caused from them drinking vital essence.
Lastly, it's possible that the Skeksis could have realized that it was killing them faster. No doubt the Emperor noticed he was dying at an accelerated rate compared to the others. The Skeksis are however so vain and so short sighted, that I imagine they cared more about how they looked in the moment and would ignore any long term effects the vital essence was having.