>A friend who is rathter pround to be a "Miss" visited friends in France recently, having just celebrated her 50th birthday. She was most upset to be called Madame rather than Mademoiselle. Apparently once an unmarried woman reaches a certain age she is called Madame out of respect, whether or not she is married. <

Yes, this is how it works. But if your friend got to 50 before it happened to her, she's doing pretty well.
I was called Madame at the age of 17 - just because I had someone else's 4 year-old child with me! And even without a child in tow, I've been called Madame for the last year or two, although I'm only 33.