Young adult fiction (YA) is a category of fiction written for readers from 12 to 18 years of age. While the genre is primarily targeted at adolescents, approximately half of YA readers are adults. The subject matter and genres of YA correlate with the age and experience of the protagonist. The genres available in YA are expansive and include most of those found in adult fiction. Common themes related to YA include friendship, first love, relationships, and identity. Stories that focus on the specific challenges of youth are sometimes referred to as problem novels or coming-of-age novels. Young adult fiction was developed to soften the transition between children's novels and adult literature. History Beginning The history of young adult literature is tied to the history of how childhood and young adulthood has been perceived. One early writer to recognize young adults as a distinct age group was Sarah Trimmer, who, in 1802, described "young adulthood" as lasting from ages 14 to 21. In her children's literature periodical, The Guardian of Education, Trimmer introduced the terms "Books for Children" (for those under fourteen) and "Books for Young Persons" (for those between fourteen and twenty-one), establishing terms of reference for young adult literature that still remain in use. Nineteenth- and early twentieth-century authors present several early works that appealed to young readers, though not necessarily written for them such as Lewis Carroll, Robert Louis Stevenson, Mark Twain, Francis Hodgson Burnett, Edith Nesbit, JM Barrie, L. Frank Baum, Astrid Lindgren, Enid Blyton, and CS Lewis. 20th century Though young adult literature had existed since at least Laura Ingalls Wilder's Little House series, which was published in the 1930s, teachers and librarians were slow to accept books for adolescents as a genre. The Heinlein juveniles were science fiction novels written by Robert A. Heinlein for Scribner's young-adult line, beginning with Rocket Ship Galileo in 1947. Scribner's published eleven more between 1947 and 1958, but rejected the thirteenth, Starship Troopers. That one was instead published by Putnam. The intended market was teenage boys, but a fourteenth novel, Podkayne of Mars (1963), featuring a young girl as the protagonist, is sometimes listed as a "Heinlein juvenile", although Heinlein himself did not consider it to be one. In the 1950s, The Catcher in the Rye (1951) attracted the attention of the adolescent demographic although it was written for adults. The themes of adolescent angst and alienation in