Matthew Poole and continuators on Genesis 5:32

32. And Noah was five hundred years old: And Noah begat Shem, Ham, and Iapheth.

(i. e.) He began to beget, God in mercy denying him Children till that time that he might not beget them to the destroyer, that he might have no more than should be saved in the Ark. Or, having before that time begotten others who were now dead, and having the approaching Flood in his view, he began again to beget a seminary for the World. Of these three Sons here following, the eldest seems to be Iaphet, Gen. 10. 21. The second was Sem, as appears because he was but an hundred years old two years after the Flood, Gen. 11. 11. The youngest Ham, Gen. 9. 24. But Sem is first named in order of dignity, as being the progenitor of the Church and of Jesus Christ, and because he and his progeny is the principal subject of this whole History.