If I remember correctly, most of the breast cancer that develops early is fast-spreading, while most of the cancer later in life is slow-spreading. So as you screen younger and younger people, you quickly reach a point where you cause more harm than good. You're only finding more disease you can do nothing about, and the false positive cause tremendous hardships on tens of thousands of women.
I also seem to remember that there's some serious discussion over the idea of categorizing cancer based on where it's first detected or starts. Cancer is much more a cell-based disorder than a location-based one. We may have created this huge categorization and treatment system that describes the problem in an inefficient way.
Good read.