Perhaps thirty years ago, many businesses in the U. S. found it difficult in addressing Young Women, either in verbally or in correspondence. But, the origination of the title “MS” made it all that more easy. Obviously, when addressing a Young Child, we can still resort to “Little Girl”, “Miss” or “Young Lady”; but, we do not set formal rules on such from on high.
Some time ago, an ENT physician (a Frenchman) told me that, in France, Little Girls we now addressed as “Madam”. An article from the NY Times is linked as follows, http://www.nytimes.com/2012/02/23/world/europe/france-drops-mademoiselle-from-official-use.html?pagewanted=print. I believe that he was merely being facetious since, in the U. S., we don’t refer to Little Girls as Ms. But, I can only imagine many older French shaking their heads.
Language is meant to be a living, breathing thing, which changes over time. Unfortunately, some of the European Countries tend to formalize it, and that’s where the problems can occur. That means that when changes are necessary, it can often be more delayed, and harder to accept.
In today’s World, where most Women are not necessarily married by 18 or so, it makes good sense to get rid of the difference between Miss and Missus (A/K/A Mrs.). But, since our culture is more colloquial, everyone still has the freedom to decide for themselves.