Misspent Youth by Peter F. Hamilton

It is forty years in 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 father of the datasphere (which replaced the Internet) and philanthropist extraordinaire. After 18 months in a German medical facility, the 78-year-old patient returns home looking like a healthy 20-year-old.

"Misspent Youth" follows the effect his reappearance has on his friends and family -- his young ex-model wife Sue, his teenage son Tim, and his long term pals, themselves all pensioners, who are starting to resent what Jeff has become.