There are no official requirements for contemporaneity.

There are two definitions for 'contemporary,' according to Merriam-Webster's. The first is, as follows: "happening, existing, living, or coming into being during the same period of time." The other delineates things that are "marked by characteristics of the present period." In regard to literature, the latter definition is most helpful. 

When we talk about contemporary literature, we are talking about those works and authors who discuss how we live now. One of the reasons why the works of Jonathan Franzen are so popular, for example, is because he writes about how people's lives are contextualized by contemporary events, such as the Iraq War or environmental destruction. 

Contemporary works are different from works that we define as "timeless," that is, those pieces that are relevant in any social context, or at any time. The works of Shakespeare most obviously fall into this category. Because his plays deal with very broad themes, such as abuse of power, tragic romances, and existential angst, he remains relevant. An author whom we read now might not be relevant one hundred years from now. To be "contemporary" means to be relevant in the moment.

What defines contemporary authors and their works as contemporary?

A "contemporary" work of literature simply means that it has been written in contemporary times--that is, in the very recent past.

I suppose one could debate how recent that past needs to be to be considered contemporary.  If a book was written 5 years ago, is is it contemporary?  What about 10 or 15 years ago?  The answers to these questions is subject to personal opinion.

Another factor might be the topic and style of the book.  If an author in 2010 writes a book that is a copy of the style of Charles Dickens or William Shakespeare, it might not be considered contemporary.  (It also probably wouldn't be a very interesting or valuable book; it's hard to imagine anyone beating Dickens or Shakespeare at their own game.)