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I was taught that Ms is appropriate when you don't know the subject's marital status or when writing a business letter.
Also, for a married woman who keeps her maiden name, neither Miss nor Mrs seems appropriate.
Being a Northerner I never used ma'am as a kid, but having lived all over the U.S. I've picked it up.  I use it regardless of age, with the exception of children of course.
Interesting . . . Ma'am makes grown women feel old; but makes young girls feel grown up!
around here everyone is a ma'am or sir, even when a girl is being scolded a lot of the scolder wills tart off by saying " No ma'am you may not!" or "No ma'am you get right back here!" same with the boys " No sir, i don't think so, you go sight to time out!" " No SIR! Un uh! you do not behave that way!"