I don't think Keturah was the same person as Hagar. If Keturah was just another name for Hagar, then she was having children at over sixty years of age. It's more likely that they were two different women.

Abraham was 140 years old when Sarah died, and his marriage to Keturah appears to have happened after that. If so, then there was probably more than one hundred years age difference between them. There was a lesser age difference between Jacob and his two wives, Rachel and Leah, but still more than 40 years.

Those men lived much longer than we do, but that was still a pretty big gap. There's nothing morally wrong with a man marrying a much younger woman, depending on his health and how long he expects to live after the marriage. Any qualms we have are in our own minds and cultural prejudices.