Misspent Youth — Peter F. Hamilton

It is forty years into the future and, following decades of research and trillions of euros spent on genetics. Europe is finally in a position to rejuvenate a human being. The first subject chosen for treatment is Jeff Baker, the creator of the Datasphere (which replaced the Internet) and philanthropist extraordinaire. After eighteen months in a German medical facility, the seventy-eight-year-old patient returns home looking like a healthy twenty-year-old.

Misspent Youth follows the effect his reappearance has on his family and friends -- his considerably younger ex-model wife Sue, his teenage son Tim, and his long-term pals, now themselves all pensioners, who start resenting what Jeff has become.