They do not die of old age, but instead are killed once the rock growing on their body becomes too heavy for them to carry (much like giant lobsters in the real world). The rocks on their backs contain their lungs, so attempts to chip away at the rock to keep them alive longer will instead kill them. However, this can be circumvented by having their body hung on a rack, so that the rocks on their backs are unable to crush them, as seen by the ancient goebie during the Call of the Ancestors quest.