Or HaChaim on Genesis 21:2/אור החיים על בראשית כ״א:ב׳

1 ותהר ותלד שרה לאברהם בן. Sarah became pregnant and bore a son for Abraham. The Torah needed to mention the pregnancy because we had learned that Sarah did not have a womb. This shows that G'd performed an additional miracle for her (Bereshit Rabbah 53,5). The reason the Torah underlines לאברהם is to make it clear that only this son is to be viewed as an extension of Abraham, not Ishmael, as we read later in 21,12. Even though the Torah described Hagar as bearing a son "for Abraham" (16,15), the fact is that at that time Abraham was still called Abram. Once his name was changed, Abraham was no longer to be referred to by his original name. This means inter alia that Ishmael was not a son of Abraham. Perhaps the statement in Berachot 13 that someone who calls Abraham Abram is guilty of violating a positive commandment is precisely for that reason. Our sages do not want Ishmael characterised as a son of Abraham. Such considerations did not apply to Jacob whose name was changed to Israel as he could comfortably identify with all his sons. I have written some more about this subject on 35,10.

2לזקוניו. In his old age. The Torah adds this word and does not content itself with the more appropriate לזקנתו, because, as I wrote earlier, there was an element of the divine soul missing in Isaac at the time he was born. Isaac had only been equipped with a soul which originated in the female emanations. The repetition of the words denoting birth in verses 2 and 3, are an allusion to the eventual birth of another soul during another stage of Abraham's old age. [לזקוניו, literally: "in his old ages." Ed.] In order for someone in this world to receive something, such as an additional dimension to his soul, it is essential that the one from whom the something new is imparted has prepared himself suitably prior to imparting it. This is a concept that kabbalists are familiar with. If Isaac had possessed only the "female" soul at the time of the עקדה, the fact that he himself was ready to receive an additional soul would not have sufficed. The Torah wrote here ותלד…לזקוניו, to indicate she gave birth "twice." When Abraham offered his son on the altar this was his preparation for the "second" birth of Isaac.

3A simpler explanation of the plural לזקוניו, would see in that word the old age of both Isaac's father and mother respectively. This would reflect Sarah's having spoken about both אחרי בלותי…ואדני זקן (18,12).