Well there is no exact answer to that question for there are many fabulous young writers out there! If you want to narrow it down you need to say what country, what genre the book is etc. There are popular books written by 15 year olds and rarely under. I'm sorry I cannot answer your question the way you probaly wanted but you need to narrow these things down for an answer.
King Tutankhamen was the most popular Pharaoh in Ancient Egyptian times, and the youngest.
Enid Blyton
Wayan Sadra-writer Pande Made Sukerta -writer Jose Ramos-Horta-Timorese activist
no one of those is the youngest writer "Sara Mohamed Elsayed " is the youngest Egytian novelist she wrote "Legends Of The Castle Of Cazesto" and "Devilish Humen"
Pliny the younger a famous writer who watched Pompeii
yes , he was very popular writer in bengali.
Eric Wright is a Canadian writer, though born in England, of mystery novels. His most popular novels are known collectively as the Charlie Salter Mysteries.
Sarah Lonsert is she won a contest look it up!!! :)
Dorothy straight was 4 years old when she wrote 'How the world began' in 1962.
One of the youngest Pakistani writers is Hareem Shah, who gained recognition for her book "One Hundred Truths" at the age of 19.
The youngest songwriter is difficult to determine definitively as it depends on various factors such as the definition of "songwriter" and the specific age of the individual in question. However, some notable young songwriters include Mozart, who composed his first piece at age 5, and Alma Deutscher, a contemporary composer who wrote her first opera at age 7. Ultimately, the title of the youngest songwriter may vary depending on individual circumstances and definitions.
I believe Laura Ingals (Little House on the Prairie) was the youngest. Anne Frank (The Diary of Anne Frank) did not set out to be a writer; her father published her diary after her death and the finish of World World II.