Problem: As early as Genesis 17, Abraham “laughed” when God told him he would have a son (Isaac) by Sarah, since he was “a hundred years old” (v.17). But here in Genesis 25, many years later, he has children by Keturah, the wife he took after Sarah died (vv. 1–2).
Solution: There are two possibilities here, either one of which would explain this difficulty. First, the Genesis 17 text does not say Abraham laughed because he knew he was too old to have children, but because Sarah was past childbearing age (cf. 17:17; 18:12). There was no sure way for a man in ancient times to know he was no longer fertile, as there was for a woman when her periods ceased. Since Abraham was only 100 here, and he lived to be 175, it is reasonable to assume that he was still fertile. By comparison, men who live till 80 today are still fertile in their 60s.
Second, even if it took a miracle on Abraham (as well as on Sarah) to restore fertility, there is no reason that his fertile state could not have lasted for many years into the future. Once animated, his virile powers could have lasted for decades. After all, he lived 75 more years. In any event, the imagined contradiction here is simply not established.