Adam Clarke on Genesis 18:10

I will certainly return - Abraham was now ninety-nine years of age, and this promise was fulfilled when he was a hundred; so that the phrase according to the time of life must mean either a complete year, or nine months from the present time, the ordinary time of pregnancy. Taken in this latter sense, Abraham was now in the ninety-ninth year of his age, and Isaac was born when he was in his hundredth year.