Complex sentence: Something of a prodigy, Rowe began writing, most likely unbeknownst to her parents, at the age of 12, and at 19 began a correspondence with John Dunton, a bookseller and founder of the Athenian Society.
Split into simpler sentences: Something of a prodigy, Rowe began writing at the age of 12, probably without her parents' knowledge. At 19 she began a correspondence with John Dunton, a bookseller and founder of the Athenian Society.