Genesis 5:32

בן חמש מאות שנה FIVE HUNDRED YEARS OLD — R. Judan said: “How is it that all the previous generations had children at the age of hundred years and this one (Noah) at the age of 500 years? The Holy One, blessed be He, said, “If they (Noah’s children) be wicked, they will perish by the water of the Flood and it will grieve this righteous man; if they be righteous, I shall have to trouble him to construct several arks.” He therefore restrained him from having children until he was 500 years old, in order that Japhet, the oldest of his sons (cf. Rashi on Genesis 10:21) should not reach the punishable age before the Flood. For it is written (Isaiah 65:20), “For as a lad shall one die when he is one hundred years old”, meaning, that in the future time, he will only then be of punishable age, and this was so, too, before the giving of the Torah (Genesis Rabbah 26:2) . את שם ואת חם ואת יפת SHEM, HAM AND JAPHETH — But was not Japheth the eldest? But you naturally first interest yourself in Shem who was a righteous man, who was born circumcised (a sign of righteousness) and from whom Abraham was to descend (Genesis Rabbah 26:3).

וַֽיְהִי־נֹ֕חַ בֶּן־חֲמֵ֥שׁ מֵא֖וֹת שָׁנָ֑ה וַיּ֣וֹלֶד נֹ֔חַ אֶת־שֵׁ֖ם אֶת־חָ֥ם וְאֶת־יָֽפֶת׃

And Noaĥ was five hundred years old: and Noaĥ begot Shem, Ḥam, and Yefet.