Genesis 21:2

For Sarah conceived ,.... This explains what is meant by the Lord's visiting her, and doing to her according to his word, see Hebrews 11:11 ,

and bare Abraham a son in his old age ; which circumstance is remarked, that the favour might appear the greater, and the more wonderful; or, "unto", or "for his old age" F14 לזקניו "pro senectute ejus", Vatablus; "senectuti ejus", Junius & Tremellius. , for the comfort of him in his old age, who having lived so many years under the promise of a son, and in the expectation of one, even of the promised seed, from whom the Messiah should spring, now has one, than which nothing could yield him greater consolation:

at the set time of which God had spoken to him , Genesis 17:21 ; God was not only faithful in fulfilling his promise, but in keeping the exact time of it.