DTO classes, represent foreign keys on multiple values

I have a question about DTO class representation. I have two tables on my database, where one has two foreign keys on another one, for example:

book(id, author_name, author_age)
author(name, age, telephone)

where book author_name and author_age are foreign keys on author name and age.

Generally what's the best way to implement this situation on a DTO class? Inside Book DTO, it's better to do something like:

public class Book {
    private String id;
    private Author author;
}

or something like

public class Book {
    private String id;
    private String author_name;
    private int author_age;
}

?