Objections have been raised to the view that Abraham married Keturah only after Sarah was dead, because of the difficulty of explaining how Abraham could beget children in his extreme old age. How could he who needed to be strengthened at the age of one hundred to father Isaac, and could only have a child by God’s miraculous intervention; how could be then beget several other children in an apparently natural way at an even more extreme age? Some conclude that although the narrative separates the two events, in fact Keturah was taken as wife at the same time as Sarah. But if Keturah’s children were conceived naturally, then it must have been before Isaac was conceived when ‘his own body [was] now [as good as] dead’ (Romans 4:19). Abraham was about eighty-six when Ishmael was conceived, and this must be near to an upper limit on the age at which he could conceive naturally. But if other children already existed, then, given his desire to have a seed in fulfilment of the promise, why were these children of Keturah not mentioned (Genesis 17:18)? Sarah would not have suggested Abraham take Hagar if other children were already alive. The suggestion that Moses reordered events here is speculation without any solid basis, and sullies Abraham’s character unnecessarily. The best explanation is that the change that God worked in him in order for him to beget Isaac had some permanent effect on his body so that he could later bear other children naturally.
But was it right for Abraham to remarry? Evidently if he was able to bear another six children he was not as decrepit as some imagine. The Scripture does not forbid remarriage after the death of a wife. Who is to say what need he had of companionship in living the itinerant life that God had called him to?