Once Loki is chained in the cave with a serpent dripping venom over him, he is destined to remain there trough the ages until the day of Ragnarok.
But in other parts of Norse myth, it's made clear that the gods must eat Idun's apples from time to time or they grow old and weak.
How does Loki survive through the ages without eating those apples? And when Ragnarok finally comes, how is he fit enough to go into battle and kill other gods like Heimdall?
Loki isn't necessarily going without Iðunn's apples. Although it is not specified anywhere, it seems reasonable to assume that his wife Sigyn is feeding him. The Gylfaginning states that Loki's torment in the cave is lessened because:
Sigyn, his wife, stands near him and holds a basin under the venom-drops; and when the basin is full, she goes and pours out the venom, but in the meantime the venom drips into his face.
Presumably, Sigyn can also bring him the apples. If she's willing to stand there all day keeping the venom from Loki's face most of the time, she's probably happy to keep him youthful.
In the Poetic Edda, it's clear that Loki is different from the Aesir. He descended from Ymir, whereas the Aesir did not. It's possible that his rules are different from the Aesir's, too. For example, he shape shifted into the female gender on several different occasions; whereas, the Aesir were never known to do that. It seemed likely that he went for several years without eating an apple and he couldn't care less.
