From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Νέος (Strong'S #3501 — Adjective — neoteros — neh'-os, neh-o'-ter-os )

for which see No. 1, above, occurs in  Luke 15:12,13;  22:26;  1—Timothy 5:1 ("younger men"); 5:2, feminine;   1—Timothy 5:11 , "younger (widows);"  1—Timothy 5:14 , "younger (widows)," RV, marg. and AV, "younger (women)" (see WIDOW);  1—Peter 5:5 . For  Titus 2:6 see Young , No. 1.

2: Ἐλάσσων (Strong'S #1640 — Verb — elasson — el-as'-sone, el-at'-tone )

is rendered "younger" in  Romans 9:12 : see Less.

King James Dictionary [2]

Younger a. Comp. Yunger. Not so old as another. A person of ninety years old is younger than one of a hundred, though certainly not a young man, nor in the first part of life.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [3]

Under the Jewish dispensation it was frequently the will of God to prefer the younger sons before the elder, notwithstanding the right of primogenitureship, as Shem before Japheth, Isaac before Ishmael, Jacob before Esau, Joseph, Judah, and Levi, before Reuben, Ephraim before Manasseh, Moses before Aaron, and David before all his brethren. In some of these cases the elder had forfeited his right of primogenitureship by transgression, as Esau and Reuben, but not so the others. The cause of the proceeding of God's providence may be conjectured to have been twofold- first, as a memorial of the sin of Cain, first-born of Adam, by which Seth and his posterity were preferred before them; and, secondly, as a type of the future preference of the Christian, or younger Church, before the Jewish, or elder Church, in consequence of the forfeiture of the latter by unbelief. See AGE.