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.
It is subjective to determine the youngest and most popular writer as opinions may vary. However, notable young writers such as Rupi Kaur, Junot Díaz, and Zadie Smith have gained significant popularity in recent years for their storytelling and unique literary voice.
It is infact Rhoald Dhal writer of the twits and charlie and the chocolate factory.
It might be Dr. Seuss or Mary Pope Osborne. They both have very good books. I have read them all.
King Tutankhamen was the most popular Pharaoh in Ancient Egyptian times, and the youngest.
Enid Blyton
The youngest song-writer that everyone in Singapore have known is Glee Lawreez D. Cagas, she is from Surigao City. She is a high school student of Caraga Regional Science High School, Surigao City, Philippines. She is a guitarist , a song-writer, a violinist, pianist and a singer.
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"
yes , he was very popular writer in bengali.
Pliny the younger a famous writer who watched Pompeii
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.
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.