Who was the youngest First Lady of the USA?

Answer 1

Frances Clara Folsom Cleveland Preston remains the youngest wife of a sitting president

Frances Clara Folsom Cleveland Preston married to the President of the United States Grover Cleveland at age 21 and becoming youngest First Lady of the USA.
Frances Folsom, who was 21 years old, married President Grover Cleveland, age 49, on June 2, 1886. Their age difference of 28 years is the second largest of any presidential marriage.