Jamieson, Fausset and Brown on Job 5:26

26. in a full age--So "full of days" (Job 42:17; Ge 35:29). Not mere length of years, but ripeness for death, one's inward and outward full development not being prematurely cut short, is denoted (Is 65:22).

Thou shalt come--not literally, but expressing willingness to die. Eliphaz speaks from the Old Testament point of view, which made full years a reward of the righteous (Psa 91:16; Ex 20:12), and premature death the lot of the wicked (Psa 55:23). The righteous are immortal till their work is done. To keep them longer would be to render them less fit to die. God takes them at their best (Is 57:1). The good are compared to wheat (Mt 13:30).

cometh in--literally, "ascends." The corn is lifted up off the earth and carried home; so the good man "is raised into the heap of sheaves" [Umbreit].