The Golden Age — John C. Wright

The Golden Age is 10,000 years in the future in our solar system, an interplanetary utopian society filled with immortal humans.

Phaethon, of Radamanthus House, is attending a glorious party at his family mansion celebrating the thousand-year anniversary of the High Transcendence. There he meets an old man who accuses him of being an imposter, and then a being from Neptune who claims to be an old friend. The Neptunian tells him that essential parts of his memory were removed and stored by the very government that Phaethon believes to be wholly honorable. It shakes his faith. Is he indeed an exile from himself? He can't resist investigating, even though to do so could mean the loss of his inheritance, his very place in society. His quest must be to regain his true identity and fulfill the destiny he chose for himself.