Jamieson, Fausset and Brown on Daniel 5:31

31. Darius the Median--that is, Cyaxares II, the son and successor of Astyages, 569-536 B.C. Though Koresh, or Cyrus, was leader of the assault, yet all was done in the name of Darius; therefore, he alone is mentioned here; but Da 6:28 shows Daniel was not ignorant of Cyrus' share in the capture of Babylon. Is 13:17; 21:2, confirm Daniel in making the Medes the leading nation in destroying Babylon. So also Jr 51:11, 28. Herodotus, on the other hand, omits mentioning Darius, as that king, being weak and sensual, gave up all the authority to his energetic nephew, Cyrus [Xenophon, Cyropædia, 1.5; 8.7].

threescore and two years old--This agrees with Xenophon [Cyropædia, 8.5,19], as to Cyaxares II.