Bible translations differ on Gen 10:21. Some refer to Shem as the oldest of Noah's sons and some refer to Japheth as the elder. Why is there a discrepancy?

You will notice that the birth order of Noah’s sons is given in 9:18: Shem, Ham, and Japheth. Now, in chapter 10, Moses traces the descendants of these sons in reverse order: Japheth (10:2ff.); Ham (10:6ff.); and Shem (10:21ff.). This reversal is noted by Harper’s Commentary, and explained by Sailhamer.

Whatever its original intent, in its present context Genesis 10 continues the segmented genealogy of Noah (Gen. 5:32; 9:28-29). Untypically, it moves from Japheth, the youngest¹ of Noah’s sons (vv. 1-5), through Ham (vv. 6-20), to Shem (vv. 21-31), the oldest son. The movement from the youngest son with his descendants to the oldest focuses attention on the genealogical line of descent in which the narrator is interested. Mays, James Luther, Ph.D., Editor, Harper’s Bible Commentary, (New York: Harper and Row, Publishers, Inc.) 1988.

John Sailhamer (The Pentateuch As Narrative -- Zondervan, 1992; pp. 136-137) has an insightful comment. The reversal is purposeful, and Sailhamer suggests why:

Here the author’s aim is to show that God’s promise concerning the seed of the woman cannot be thwarted by the confusion and scattering of the nations at Babylon. Though the seed of Noah were scattered at Babylon, God had preserved a line of ten great men from Noah to the chosen seed of Abraham. Out of the ruins of two great cities, the city of Cain and the city of Babylon, God has preserved his promised seed. By beginning the list of names over again with Shem, the author shows his intention to bypass the other line that had been traced to Shem in the previous chapter (10:26-30).

Your problem seems to be that verse 21 is translated differently, depending upon the translation.

KJV Genesis 10:21 Unto Shem also, the father of all the children of Eber, the brother of Japheth the elder, even to him were children born.

ASV Genesis 10:21 And unto Shem, the father of all the children of Eber, the elder brother of Japheth, to him also were children born.

NIV Genesis 10:21 Sons were also born to Shem, whose older brother was {21 Or Shem, the older brother of} Japheth; Shem was the ancestor of all the sons of Eber.

NAS Genesis 10:21 And also to Shem, the father of all the children of Eber, and the older brother of Japheth, children were born.

NRS Genesis 10:21 To Shem also, the father of all the children of Eber, the elder brother of Japheth, children were born.

NLT Genesis 10:21 Sons were also born to Shem, the older brother of Japheth. <1> Shem was the ancestor of all the descendants of Eber.

NJB Genesis 10:21 Shem too fathered sons, being ancestor of all the sons of Eber and Japheth’s elder brother.

The difficulty is that translators have understood the Hebrew text of Genesis 10:21 differently. Since we know from 9:18 that Shem is the oldest brother, the translation of the New American Standard (with a marginal note, giving the literal rendering) is most accurate. The New Living Translation is certainly the easiest and simplest to read.

¹ Editor’s Note: This article takes the order of sons in Gen. 9:18 as primary, and focuses on explaining the seeming discrepancy of who was the firstborn as might be challenged by the different interpretations of Gen. 10:21. An additional question could be raised on the identity of the second born from Gen. 9:24. The comparative Hebrew word there could be translated as “younger” or “youngest” (cf. KJV, NKJV, vs. NET, NASB, ESV, NIV, etc.). Fausset’s Bible Dictionary is also helpful for comparison on this point. Ham’s son Canaan may be the referent, or it might simply be a reference to Ham being younger than Shem.