I read this book maybe five or six years ago and recently remembered the strange story in it and I can't remember the title or author to find it again.
Here's what I remember of the story: this old guy is tasked with finding the fountain of youth and to do so must sail up the Nile to the Mountains of the moon.
On the way there they pick up a young girl, who they find in a papyrus field or forest of some kind, apparently wild. She tags along with the group (the girl and the guy are the main protagonists). The guy is some kind of enlightened sage/mystic/philosopher.
It takes them ages but they finally arrive at the mountains and there's some really weird cult there to do with like stem cell regeneration (all this is set in like ancient Egypt so totally sci-fi stuff) and (now this is where it gets really weird) the old guy who happened to be an eunuch gets cured (if you catch my drift).
He spends ages at this monastery place healing and stuff and then sets off again to find the fountain. He does so eventually, and bathes in it and gets rejuvenated.
He comes down off the mountain and the girl who they'd found has grown up and basically, they end up together because he's now young and immortal.
They sail off together into the sunset and make it their life's work to find the fountain again so that the girl can be eternally young as well if I remember correctly they succeed and live happily ever after.
As you can see this story is super strange and for the life of me I can't remember the book but the story really stuck in my head. I'm pretty sure the author is male and that the cover of the book is white with gold embossed Egyptian symbols.