Mary Shelley was 18 years old when she began writing Frankenstein, and she completed the first draft around 1816, at about age 18. The novel was published anonymously in 1818 when she was about 20 or 21, depending on the exact publication date and how one counts the drafting period. Most commonly cited: born August 30, 1797; Frankenstein started in 1816 at age 18; published in 1818 (age 20).