17. age--days of life.
      the noon-day--namely, of thy former prosperity; which, in the poet's image, had gone on increasing, until it reached its height, as the sun rises higher and higher until it reaches the meridian (Pr 4:18).
      shine forth--rather, "though now in darkness, thou shall be as the morning"; or, "thy darkness (if any dark shade should arise on thee, it) shall be as the morning" (only the dullness of morning twilight, not nocturnal darkness) [UMBREIT].

JFB.