According to Wikipedia
Young adult fiction (YA) is a category of fiction written for readers from 12 to 18 years of age. While the genre is targeted to teenagers, 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.