In the case of children and young adults books, the audience is mandatory if you select BISAC classifications specific to children (all those beginning with Juvenile or Young adult). If you select such categories without specifying an audience, your book may be poorly referenced, and even rejected, by some retailers.
In the warehouse, under the tab Audience>Age group of interest, you can set only the minimum age (e.g. from 12 years old) or a specific age range (e.g. from 9 to 12 years old). The Maximum age field is therefore not mandatory.
