How old was S. E. Hinton when she wrote The Outsiders?

Susan E. Hinton was all of 15 years old when she completed
writing her groundsbreaking teen novel, The Outsiders. Susan was a
sophomore at Tulsa, Oklahoma's Will Rogers High School when she began work on the novel.
Her father, Grady, was suffering from a brain tumor and, according to her
mother,

...
the more ill her husband became, the harder Hinton worked on her
writing.

Susan's father died
during her junior year at Will Rogers, the same year she finished the book. After four
drafts of the novel, Susan still had no hope of The Outsiders ever
being published; however, she apparently allowed a friend's mother--a writer of
children's books--to read it, and the woman steered Susan toward an agent in New York.
The book was published during her freshman year of college, and it was decided to use
only her initials: Publishers believed that male readers would not accept the reality of
the novel if they knew it was written by a woman.