Mrs. is a title used before a married woman’s name (e.g., “Mrs. Carlton”). It shouldn’t be used for an unmarried woman, regardless of her age. It originated as an abbreviation of “Mistress,” but it’s now pronounced [miss -iz] and should be written in its abbreviated form, not as “Mistress.”
Old: Directed by M. Night Shyamalan. With Gael García Bernal, Vicky Krieps, Rufus Sewell, Alex Wolff. A vacationing family discovers that the secluded beach where they're relaxing for a few hours is somehow causing them to age rapidly, reducing their entire lives into a single day.
Where old expresses an absolute, an arrival at old age, older takes a more relative view of aging as a continuum—older, but not yet old. As such, older is not just a euphemism for the blunter old but rather a more precise term for someone between middle and advanced age.
An old person has lived long, nearly to the end of the usual period of life. An aged person is very far advanced in years, and is usually afflicted with the infirmities of age.
There are 40 meanings listed in OED's entry for the adjective old, two of which are labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.
The contractions Mr. and Mrs. are short for Mister and Missus/Missis. These contractions, like their longer forms, are used in etiquette to show respect to men and women.
Mr., Mrs., Miss, and Ms.: What They Mean And How To Use Them
Mrs. originated as a contraction of the honorific Mistress (the feminine of Mister or Master) which was originally applied to both married and unmarried women in the upper class. Writers who used Mrs for unmarried women include Daniel Defoe, Samuel Richardson, Henry Fielding, and Samuel Johnson.