Misspent Youth by Peter Hamilton

Jeff Baker is a famous philanthropist and the inventor of technology that replaced the internet with a 'datasphere'. He is the focus of Europe's groundbreaking new project: an attempt to rejuvenate a human being. After 15 months in a medical facility Jeff is released looking like a healthy 20 year old; he is 78.

Not to be compared with the Night's Dawn trilogy, which was breathtaking, but entirely readable.