Final answer:
Mary Shelley was 20 years old when she published Frankenstein in 1818.
Step-by-step explanation:
Mary Shelley was just 20 years old when Frankenstein was published in 1818. Shelley was an influential figure of the Romantic movement, married to Percy Bysshe Shelley, a noted poet of the era, and the daughter of two prominent thinkers, Mary Wollstonecraft and William Godwin. Much of her life experience, including the relationships with her husband, fellow writers, and her own radical parents, influenced her in writing her masterpiece, Frankenstein, amid the backdrop of a life spiked with scandal, tragedy, and intellectual fervor.