Well she was almost of "granny age" when they married.
Actually, the marriage of Bess and Harry Truman was a very touching love story.
He wooed her for about twenty years, and worked hard to have something to offer her as a man.
They married relatively late in life.
Read David McCullough's biography Truman and you will be surprised at their love affair.
My stack o' books to read is already huge, but I'll take your recommendation of Truman under advisement.