Haha. This has turned into a discussion of, as Mo puts it, the age-skirt-length-police.
For the record, I wear short skirts, and I'm 41. I don't wear them when I want to look sophisticated, though. I also don't wear ones as short as the one Sally's wearing that MaryK linked to. I agree that that one's a long tunic, although with the very heavy tights and boots she's wearing, I don't think she looks indecent. As to age appropriateness -- well, that's a matter of opinion, as the comments demonstrate!
Gigi, I just realized I didn't answer part of your question. Yes, absolutely, the type of skirt makes a difference. A super-short, flippy a-line skirt has a much greater potential to look silly (and cause inadvertent flashing of the lady parts) than a structured straight one! I don't know if the shape of the skirt has an effect on the golden rule though. I guess it might.
As to Sally's comment about mid-thigh skirts messing with the golden rule or something -- I don't think she can make a generalization about that. Sure, that hem length may muck with her proportions, but it probably doesn't muck with yours. Sally's thighs are much fuller than her calves, so that each leg forms an inverted cone shape. I can see how cutting them off halfway up the thigh would be less flattering on her. You, on the other hand, describe yourself as a short, slender gamine. Sounds like your legs are made for mid-thigh mini skirts.
I also don't totally trust Sally's judgment when it comes to proportions and figure flattery, so I'd take what she says with a grain of salt.