The genre of young adult fiction is a relatively newly defined literary genre, even though the style has existed for centuries. YA fiction is generally aimed at younger audiences between the ages of 12 and 18, although it has been estimated that possibly around half of all YA readers are adults.
YA novels tend to focus on issues which are experienced by the intended audience, mainly revolving around issues of puberty and bodily growth, problems with parents and society, and early romantic encounters. Typically, YA novels feature a protagonist who has to undergo personal growth to defeat an obstacle that could not be previously overcome. A notable example from recent times is Harry Potter from the Harry Potter series, who has to overcome a series of hurdles and go through much personal growth to defeat the villain of the series.
There have been several books which were not initially written for young adults but became incredibly popular with that group. The Catcher in the Rye is a notable example, as the book became popular with young adults unintentionally, most likely due to the age of the protagonist in the story.
Currently, most authors tend to target a particular age group while writing their stories, as this can help when communicating with publishers when attempting to sell their stories. Although books aimed at adults can become popular with young adults, particularly those over the age of 16, most of the themes which adults tend to prefer are more relatable to those over a certain age.