I think for younger men, sexiness may be all about a woman's body and the more they see the sexier she appears.
However, as a man gets older, he may admire a woman in a miniskirt, but considering her sexy is another matter. A woman can be sexy at any age, if that's who she is inside. She doesn't need a mini skirt or anything all that revealing.
More often than not, younger women show more of their bodies because they haven't yet learned the mysterious, alluring qualities that an older woman can possess. Sure Lauren Bacall made it look easy, but I think if any woman thinks of herself as sexy, she won't need a mini skirt to prove it.