Thou shalt come to thy grave in a full age, like as a shock of corn cometh in in his season.
In a full age - so full of days (42:17; Genesis 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. "As the days of a tree are the days of my people, and mine elect shall long enjoy the work of their hands" (Isaiah 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 - "With long life will I satisfy him" (Psalms 91:16; Exodus 20:12), and premature death the lot of the wicked - "Bloody and deceitful men shall not live out half their days" (Psalms 55:23). The righteous are immortal until their work is done. To keep them longer would be to render them less fit to die. God takes them at their best - "The righteous is taken away from the evil to come" (Isaiah 57:1). The good are compared to wheat - "Gather the wheat into my barn" (Matthew 13:30).
Cometh in - literally, ascends. The grain is lifted up off the earth and carried home; so the good man 'Is raised into the heap of sheaves' (Umbreit).
In his season - in its right time, when the grain is fully ripe (cf. Psalms 1:3) - "He shall be like a tree planted by the waters, that bringeth forth his fruit in his season."