Shem: comes last (ace. to the Structure, which is an Introversion) because his "Generations" occupy the rest of the book. Hence when mentioned with the other two, Shem the youngest comes first. See note Genesis 5:32; Genesis 9:24; Genesis 10:1.
the elder, See Genesis 5:32. Shem (N.T. Sem) was the seed-line that Christ would come through (Luke 3:36).