The median age of woman with breast cancer in the United States in the period from 1998 - 2002 was 61 years old (for DCIS it was 58.) So in other words half the women were older or younger than this age.  So even though this cancer is generally seen in older women, the chart below makes it clear that 23% of these cancers will be diagnosed in women less than 50 and 5% in women in their 20's and 30's