And have you ever noticed that a child will call an 18-year-old who is married “Mrs. So-and-so” (or “Mr. So-and-so”), but will call a 50-year-old who isn’t married by his or her first name?
And an 18-year-old will be called a “married woman,” but a 50-year-old is a “single girl.”
IME I have not noticed any of the above.
Kids tend to call adults however the adult introduced him/herself.
Most people will refer to a 50-year-old as a woman and an 18-year-old as a girl regardless of their married status. Of course there are people who will still refer to an octogenarian as a girl and are already calling a ten-year-old a woman, but those are rare exceptions.