And Abraham was an hundred years old, when his son Isaac was born unto him.

When he was a hundred years old: for at this age of his father was Isaac born.

And Abraham is a son of a hundred years in Isaac his son being born to him,

Now Abraham was a hundred years old when the birth of Isaac took place.

when he was one hundred years old. Indeed, at this stage of his father's life, Isaac was born.

Abraham autem erat centum annorum, quando natus est ei Ishac filius suus.

And Abraham was an hundred years old. Moses again records the age of Abraham the better to excite the minds of his readers to a consideration of the miracle. And although mention is made only of Abraham, let us yet remember that he is, in this place, set before us, not as a man of lust, but as the husband of Sarah, who has obtained, through her, a lawful seed, in extreme old age, when the strength of both had failed. For the power of God was chiefly conspicuous in this, that when their marriage had been fruitless more than sixty years, suddenly they obtain offspring [1] . Sarah, truly, in order to make amends for the doubt to which she had given way, now exultingly proclaims the kindness of God, with becoming praises. And first, she says, that God had given her occasion of joy; not of common joy, but of such as should cause all men to congratulate her. Secondly, for the purpose of amplification, she assumes the character of an astonished inquirer, Who would have told this to Abraham?' Some explain the clause in question, will laugh at me,' as if Sarah had said, with shame, that she should be a proverb to the common people. But the former sense is more suitable; namely, Whosoever shall hear it, will laugh with me;' that is, for the sake of congratulating me.

Footnotes

1 - "Quod quum ultra sexaginta annos sterile illis fuisset conjugium, effoetis jam et semimortuis, subito nata est prolis."

And Abraham was an hundred years old when son Isaac was born unto him. So that this was years after his departure from Haran, and coming into the land of Canaan, for then he was seventy five years of age, Genesis 12:4; and this exactly agrees with the account of Demetrius, as related by Polyhistor, an Heathen writer (o), who makes Isaac to be born just twenty five years from Abraham's coming into the land of Canaan, and who must be now an hundred years old, being ninety nine at the time the Lord appeared unto him, and promised him a son at the set time the next year, Genesis 17:1. This is observed, both to show the wonderful favour to Abraham, and the faithfulness of God in the exact performance of his promise: according to Bishop Usher (p), Isaac was born A. M. 2108, and before Christ 1896, and probably at Beersheba, see Genesis 21:33.
(o) Apud Euseb. Praepar. Evangel. l. 9. c. 21. p. 425. (p) Annales Vet. Test. p. 9.