Is Mademoiselle french for Ms?
mademoiselle (unmarried), pl. mesdemoiselles = Miss / misses
madame (married), pl. mesdames = Mrs.
madame would be used also in business correspondance, or contexts where the marriage status of the person is not relevant, as ms.
Mme is the abbreviated form of "madame", i.e. Mrs. Melle / Melles. ("mademoiselle/mesdemoiselles) is the translation of Miss/Misses Ms. has no French equivalent - French people tend to use Mme when the marital status is irrelevant to the purpose of the letter. In spoken French we use "madame" or "mademoiselle" depending of the context: an older person will be called "madame", whereas a younger one may be called "mademoiselle"
French does not have an equivalent of the neutral female title "Ms." If you do not know whether a woman is married, use Mademoiselle for a young woman and Madame for older women. As for the approximate age where you should make the cutoff, I'm not sure--maybe 30? That is not true at all. France does not really use Ms. most women after a certain age become Madame whether they are married or not, but…