* A fictional book: Many books and movies use the phrase "Book of Secrets" for a plot device.
* A real book with that title: Many books throughout history have been called "The Book of Secrets" with varying content and authors. To understand which book you mean, I would need more context, such as the author, genre, or a brief description of the contents.
* A collection of secrets: People often use the phrase "Book of Secrets" to describe any collection of secret information, which wouldn't have a single age.
To answer your question, please provide more information about the "Book of Secrets" you are referring to.