The speaker starts out as a youth, "Drives my green age," then ages and approaches old age, "Hauls my shroud sail." You can tell this because of two things. "Green" wood is young and uncured wood and "green" is used as an idiom to mean new to something, in this case, new to ageing. "Shroud" is the funeral wrapping that at one time people were buried with. So the speaker's life is being drawn along by his approaching death. The end's in the death that was expected: "How at my sheet goes the same crooked worm." "Sheet" and "crooked worm" are idiomatic expressions that mean death.