Asked one of the female client execs I met in France and was told to call every woman of obviously marriageable age "madame" - otherwise you are implying they could not find a husband.

Mademoiselle is reserved for college students or younger.

Chère Florence: my amies parisiennes have confided that "femme d'un age certain" suggests a woman who is indeed older than "femme d'un certain age," though I also get the sense that the expression is more colloquial than standard. It seems to apply to a woman who looks/dresses/acts as if she's no longer in her 50s, or maybe 60s, but it's still a bit mysterious to this man. (No doubt as it should be.)

Preferences vary with age. The older generation seems to consider Mademoiselle pejorative because it implies that a woman couldn't find a husband. But many of the younger generation aren't looking for husbands, and find Mademoiselle a compliment because it implies that they are still young.

As for pronunciation, it's "mahd-mwah-zehl." The 'd' is pronounced, contrary to what you might have read or heard.

Knock knock
Who's there?
Madame
Madame who?
Madame foot's caught in the door!

Cher Rizzuto,

Your amies parisiennes are right in a way, but have forgotten to tell you that "femme d'un âge certain" is an euphemism (and not a very gentle one in general) for "old woman", especially one who would like to hide her actual age ;-)

rather than 'old woman' I prefer to think, 'mature'