Genesis 17:24

Don't think it was without purpose that Scripture indicated to us his age; instead, it was for you to learn from the just man's obedience in meekly submitting to pain despite his extreme old age on account of God's command, and not only he but also Ishmael and all the servants—that was the reason for giving the ages.

He was ninety-nine years old when he circumcised the flesh of his foreskin, and Ishmael his son had completed thirteen years at the time of his circumcision. Therefore, Abraham was a hundred years old, and Ishmael was thirteen years old when they were circumcised: so that, after a year, when Isaac was born, Abraham was a hundred years old, and Ishmael was fourteen, that is, twice seven, years old? Indeed, in a great mystery: because evidently Isaac held the figure of the New Testament, and Ishmael of the Old Testament. For Isaac, when he was born, his father was a hundred years old, to signify that the grace of the New Testament would bring the children of his promise to the inheritance of the heavenly kingdom. Indeed, the number one hundred, as has often been said, in the arithmetical count, passes from the left to the right, and our heavenly mother Jerusalem is rightly marked by the right hand; for those to whom it is permitted to enter it will stand on the right hand of the Judge in the last examination. In the birth of the same Isaac, Ishmael had completed two weeks of years; evidently because of the sacrament of the Sabbath, which was observed in the Old Testament; so that in the fifteenth year, as if in the sign of the resurrection, that is, of the true circumcision, Isaac would be born; where it would be typified because the passing legal observance given by Moses would be succeeded by grace and truth which came by Jesus Christ. Therefore, when Isaac, who prefigured the grace of the New Testament, was born, his elder brother Ishmael, who signified the Old Testament, had completed the number seven in his age; because the observance of the law, in which the Sabbath held the greatest place, was fit to be extended until the time of grace; but the same number seven is doubled concerning the perfection of the same sacrament; because the law of God desires us to be freed both from servile, that is, from evil works in the body, and from harmful thoughts in the mind. The number seven is doubled because the law commands rest for bodies to its followers in this life and promises rest for souls in the future life; when Isaac was born and the first year of his age began, Ishmael began his fifteenth year because, with the grace of the Gospel appearing, even the law itself, spiritually understood, was found to be full of heavenly sacraments and to have preached the…