Anna Sewell sold Black Beauty when she was 57.

Laura Ingalls Wilder, author of the Little House on the Prairie series,
didn’t have her first book published until she was well into her 60s. 

Bangladeshi writer Nirad Chaudhuri's first book, The Autobiography of an Unknown Indian,
was published in 1951, when he was 54 years old. Its sequel hit the market in 1988, when he was 90. 
And his final book, Three Horsemen of the New Apocalypse, was published in 1997—when he was 100.