This book is short, but it has a very teen sensibility. Issues of parental strife, personal identity, and finding one's place are all common to adolescence, even in a medieval setting, and Mordred's angst, while off-putting to some adults, will ring very true for middle school and high school readers. The incest and attempted murder would probably be enough for me to avoid sharing it with a child much younger than that, but obviously every parent will have different standards and every kid will have different levels of sensitivity.
Nancy Springer's writing is very atmospheric, and her prose paints a vivid picture of Mordred's home by the sea as well as of Camelot. Even more so than The Squire's Tale this book immerses the reader in the time period and helps them feel as though they have experienced the events of the story firsthand. Springer also does a wonderful job of humanizing Mordred and turning him into a regular teenage boy, which makes the entire Arthurian universe feel more welcoming to teens, and more relevant. Other characters - including the Lady of the Lake and Morgan Le Fay - are also portrayed as richly layered individuals.
This book would serve as a great introduction to Camelot for an uninitiated teen, as well as an entertaining retelling for kids who already know the major characters and story lines. There is a companion novel, I Am Morgan Le Fay, which humanizes another notorious Arthurian villain. Another read-alike for this book is the beautiful novel in verse, Song of the Sparrow.