In this story, the main character is over 400 years old, but looks about mid-thirties. He's not exactly immortal, but it's pretty close to that. The book has a plausible explanation but I won't go into that any further because....
The important thing here is that this is a great story! Like the movie The Highlander, this book makes you think about what it would be like to live on after everyone you know and love is dead. The scene in that movie with the song, "Who Wants to Live Forever" sung by Queen, still breaks my heart when the immortal Connor calls for his wife and she comes up the path, suddenly older, while he still looks just the same.
But I digress.
Anyways, this story does a great job of making such a strange concept seem real This is a great book!