Mary Shelley was 18 years old when she began writing Frankenstein. In the summer of 1816, she was Mary Godwin and was staying near Lake Geneva with Percy Bysshe Shelley, Claire Clairmont, Lord Byron, and John Polidori.
The group was confined indoors by unusually cold and rainy weather, part of the climatic disruption later associated with the 1815 eruption of Mount Tambora. After reading ghost stories, Byron proposed that everyone compose a supernatural tale. Mary initially struggled to find an idea, but she later imagined a student bringing an assembled body to life and recoiling from the result.
She began drafting the novel in 1816 and continued revising it after returning to England. The manuscript was completed in April or May 1817, when she was 19. Frankenstein was then published anonymously on 1 January 1818, shortly before her 21st birthday.
The age can be confused with the date of publication: Mary was 18 at the beginning of the composition, 19 when she completed the manuscript, and 20 when the first edition appeared.