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 Gen_5:32; Gen_9:24; Gen_10:1.
the elder, See Gen_5:32. Shem (N.T. Sem) was the seed-line that Christ would come through (Luk_3:36).