In terms of the biological onset of puberty coming for younger and younger children, this might IMHO be due to the advances in medical care and nutrition that have taken place over the last 100 years. Our society is healthier (aside from the obesity issue) and better fed than it ever has been before.
In the sexualization of younger and younger children, I think that this phenomenon is mostly due to the media and industry targeting the tweenie set as consumers more and more heavily. They've got money to spend, and there's no shortage of unscrupulous businesses selling sex as maturity that want it. What kid doesn't want to appear more "mature" than their schoolmates?
And for the record, my 13 year-old daughter shops for her own clothes, but nowhere in her entire wardrobe is anything that comes close to the slutty gear. She doesn't want to dress like that, and even if she did, there's absolutely no bloody way I'd ever let her leave the house looking like some Australian street pimp's nightmare.
Be considerate to others or I will bite your torso and give you a disease!