Alice Hathaway became Teddy's wife when she was 19. Tragically dying after giving birth to their first child two day later. And yes, Theodore was not the president at that time.
Since no one else is playing, and you're stumped George, I will just reveal the answer.
The youngest wife of a president to become First Lady was also the first to be married in the White House. Frances Cleveland was 21 yrs. old when she married Grover Cleveland in a White House ceremony. If memory serves, John Phillip Souza led the U.S. Marine band and conducted, even wrote a song.… A lot of Presidential firsts with this Presidential couple. First child born in the White House to a sitting president, from Grover's second wife. First non-consecutive terms by a First Lady. (Odd, now that Melania Trump had become First Lady again, I've yet to hear her name ever mentioned).
Frances (who liked to be called Frank) was a celebrity in her own right. America couldn't get enough of the young presidential bride. The media couldn't get enough. That's why I made the association of her with Jackie Kennedy. Like Jackie in her day, Frances took America by storm in hers.