The central theme of Number the Stars is the difficulty of growing up. One could make the case that Lowry uses the context of World War II as a way of making these difficulties stand out clearly. The novel focuses on Annemarie Johansen's personal experiences with growing up, but her experiences are common to most young people. Growing up is presented as a struggle for identity. Does Annemarie belong to the world of adults or to the world of children? Such distinctions are always difficult to make, but the situations the war creates makes these distinctions even more difficult. The roles Annemarie must play blur the line between a child's responsibilities and an adult's responsibilities. Lowry uses the war to demonstrate how confusing the separation between childhood and adulthood can be. Because of the war, Annemarie needs greater protection, but at the same time has to learn things that one normally does not learn until later life.
Annemarie is frequently compared and contrasted to other characters in the novel, particularly Kirsti and Peter Neilsen. Kirsti's complete state of innocence and stream of childish requests is juxtaposed with Annemarie's growing sense of responsibility. Innocence, perhaps the most prominent feature of childhood, is no longer possible for Annemarie. Because of this, she does not identify with her little sister. But Annemarie is not sure she belongs with the adults, either. Her observation that Peter has "taken his place in the world of adults" makes it clear that Annemarie does not feel like a member of that world. Annemarie's concerns about her ability to be brave also make her feel that she is mature. Yet she is beyond the point where her youth will protect her from being called on for help.
The role of knowledge and concealment adds to the conflict of childhood versus adulthood. Again, the war plays a part in complicating this issue. It is not appropriate for a child to be told certain things, particularly concerning war. But in order for Annemarie to process what is happening around her, she wants to know more. This curiosity is also a fundamental part of growing up. But in Number the Stars, ignorance can be a form of self-protection. So Annemarie struggles with differentiating between the information that is being withheld for her own safety and the information that is being hidden because she is so young.
Physical and mental voyages run throughout Number the Stars. The novel centers around the trip that the Rosen's and the other Jews must make across the ocean to safety. That voyage takes place in three parts: the trip from Copenhagen to Gilleleje, the walk from the house to the boat, and the final crossing to Sweden. Parallel to this voyage is the journey of Annemarie's growth. As she makes physical trips from place to place, she is developing new ideas and new ways of viewing herself. The trip she makes through the woods to deliver the packet to Henrik is particularly symbolic. The passage into the woods marks a transformation. Annemarie is taking on a job that an adult would normally perform; thus, she makes literal and figurative steps towards maturity. Peter Neilsen is another character we see transformed. His interactions with Mrs. Johansen shift. Toward the end of the novel, Peter and Mrs. Johansen become equals.
The reality of war is at times so terrible and strange that it feels unreal. Annemarie sometimes has difficulty accepting the events of the war as real. She fictionalizes them, making the war into a fairy tale reality. At other times, fairytales are contrasted to the war. For example, Annemarie says that everything has changed except the fairy tales. The fairy tales are also used as a means of showing that Annemarie is leaving her childhood behind. Kirsti loves stories about kings and queens, but Annemarie does not care for them. She even wants to correct her sister's overactive imagination at times. Despite herself, though, Annemarie finds support in the world of fiction. Fairy tales are often used as a way of explaining something that is hard to understand or cope with. So when her life becomes truly frightening or confusing, Annemarie reverts to seeing the war as if it were a fairy tale. As she goes to deliver the packet to Henrik, Annemarie makes the trip into the story of Little Red Riding-Hood. By turning her own life into fiction, Annemarie is able to deal with her fear and get the packet to her uncle. In the end, of course, reality is nothing like the fairy tales, things do not always end well, and the heroes do not live happily ever after. Peter Neilsen dies. It is revealed that Lise's death was a product of the war, too. Even Kristi grows out of fairy tales.