A few thoughts in no particular order, written by a middle-aged father of grown daughters:
1) It's amusing that we have a thread deploring fashions among teenaged girls on a site that also has several threads dedicated to jokes about blondes and another thread that has a link to a video of French maids working on a car. Now, I enjoy a joke and a French maid as much as the next guy does. But when you make women objects of mockery and ridicule, when you reduce them to physical features--blonde hair, big boobs, whatever--you're contributing to a culture that defines women in terms of their bodies and sex appeal. Do not, then, be surprised if some women follow instructions and dress in ways that they have been trained to believe will attract men. So whose fault is it that the girls at the mall look the way they do? Look in the mirror.
2) If young women are indeed dressing like sluts or hookers--more on this question in a moment--then let's try to find the real causes. To blame TV or Hollywood or music videos is, at best, to blame the immediate cause, and that's never good enough in cause-and-effect thinking. The media merely cash in on a bigger problem. We do not tell our daughters often enough that although they have bodies, they are their minds. We--parents, teachers--do not push girls hard enough to excel in math, science, and engineering; we do not make sure that our daughters know about such role models as female astronauts, female chemists, female physicists, female composers, female authors, female athletes, female political leaders, female geologists, female engineers; in short, we abandon too many girls to the imbecility of the entertainment industry and let amoral, greedy people tell our daughters that they are nothing but breasts and hips. And some of our daughters believe those amoral, greedy people because the grownups haven't shown them enough alternatives.
3) You do realize, don't you, that any link between attire and sexuality is artificial and socially constructed? In other words, clothing is neither inherently sexy nor inherently demure. In some times and places, clothing that shows a lot of cleavage has been considered provocative; in other times and places, the same amount of cleavage wouldn't have raised an eyebrow. A tight, slinky dress is the sign of sophisticated high fashion in one time and place, and the mark of a tramp in another. Polynesian women went bare-breasted, and Polynesian men didn't think less of them for it. So an outfit that you think is cheap and sluttish may well have an entirely different meaning to the young person wearing it. I teach at a community college. Many young women come to class wearing push-up bras, blouses with plunging necklines, and skin-tight jeans, short skirts, or gym shorts. Then they write essays arguing for the importance of sexual abstinence until marriage. I'm not kidding. I can only conclude that they and I have entirely different interpretations of those bras and blouses.
4) Let's not forget that one of the jobs of youth is to shock and shake up the geezers. We did it when we were young. And to a 16-year-old, a guy in his late 20s is a geezer.
5) I suspect that by the time the third generation of Homo sapiens reached adolescence, their grandparents were complaining about the decline of civilization. Young people have always shocked and appalled their elders. Back in the 1920s, old folks pointed to flappers as evidence of moral decline and decadence. Fifteen years later, those flappers were the women who went to work in factories and built airplanes and tanks during World War II. If you want to go further back in history, consult the ancient Greeks; they were certain that things had been better back in the Good Old Days.
6) A few posters have commented to the effect that "Girls didn't dress like that when I was young." Please. Gerald Ford was president when I was a senior in high school. My female classmates wore jeans that looked like a coat of denim-colored paint applied below the waist or shorts so short that they barely covered their panties. And while my students favor push-up bras (I assume; it's the only explanation I have for all the cleavage), many of the women with whom I went to college dispensed with brassieres altogether, which made paying attention to the professor very difficult during the cooler months. Styles change, but I don't think the 18-year-olds I teach are any sexier than the 18-year-olds with whom I learned.
Now, to the extent that it's happening, the sexualization of young teenagers and even pre-teens is just plain creepy and disturbing. But even in these times, a 12- or 13-year-old isn't much more than the product of the influence or neglect of grownups.