(CNN) -- Hannah Powell-Auslam of La Mirada, California, had surgery this month to check her lymph nodes, just in case the breast cancer had spread.

Taylor Thompson of Little Rock, Arkansas, also had an aggressive form of breast cancer, one that has a 98 percent chance of returning. It required surgery in June.

They're two young women fighting breast cancer. Or rather, two girls: Hannah is 11. Taylor is 13.

While Taylor and Hannah's cases are rare, they're extreme examples of a troubling trend emerging with breast cancer, medical experts say. Younger women are getting a disease that usually strikes around menopause -- and no one knows why.

"The breast is a very sensitive, vulnerable organ," said Dr. Marisa Weiss, founder of Breastcancer.org, a breast health Web site. "The breast is the only organ in men and women that is formed after you're born." Most of the breast forms during adolescence, she said. "It's while organs are formed that they are most vulnerable to changes and insults."

Breast cancer could theoretically occur at any age and for either gender.

"You have breast tissue, so at any point, if you have that tissue in your body, it can become cancer," said Dr. Jennifer Litton, an assistant professor in the department of breast medical oncology at the University of Texas M.D. Anderson Cancer Center in Houston.

Litton found that in women with the gene mutation BRCA1 and BRCA2, which is linked to breast cancer, the disease is diagnosed six years earlier than the previous generation. The cause remains unclear.

"Maybe women who have a family history, they were getting screened earlier, or having more breast self-awareness," she said. "It could be something hormonal or environmental that's causing women to have breast cancer earlier... that certainly is a big area of interest."

But cases of breast cancer occurring in the 20s, teens and tweens are outliers, said Litton, who works with young patients. Only about 7 percent of breast cancer cases occur in patients under the age of 40.

Taylor's diagnosis startled her family. They have no history of breast cancer.

"I was actually confused because I never heard of no one that young going through something like this," said Stephanie Anderson, Taylor's mother. "And for it to be my daughter at the age of 13, it was like too much at one time. I was concerned about her and how she'd react because she's so young."

Taylor had an aggressive type of cancer called a phyllodes tumor, which appears in less than 1 percent of breast cancer cases.

"I didn't know how to explain to my 13-year-old daughter that she had breast cancer, when she didn't know what it was," Anderson said.

Taylor's still in the age where she loves hot pink (especially when paired with stars) and embraces glitter lip gloss.

When she heard the news, she asked her mother: "Am I going to die? Is this going to kill me? Is my hair going to fall out? What's going to happen?"

In July, Taylor had surgery to remove the cancer. Phyllodes tumor is not very responsive to chemotherapy or radiation treatment.

After her surgery in the summer, the eighth-grader returned to middle school. Her mom said Taylor was worried about how her peers might perceive her because of the cancer.

"The other people found out and I told them myself that I had breast cancer," said Taylor. "And they were crying."

The community rallied around Taylor, holding volleyball games, wearing pink ribbons in her honor, organizing dinners and making her a grand marshal in a recent parade, her mother said.

Taylor checks in every six months at the hospital to make sure the cancer has not returned.

Weiss, a breast cancer oncologist, suggested that everyday pollutants such as chemicals including bisphenol A and dioxins could bombard hormone receptors, causing abnormalities in the breast.

Another risk factor for breast cancer is obesity. "When it comes to breast cancer risks, the extra fat in obesity leads to extra estrogen production, which can lead to extra breast cell growth, including abnormal breast cell growths," she said.

Additionally, being obese means a girl is more likely to get her first period earlier. The longer a woman has her menstrual cycle, the greater the risk of getting breast cancer, medical experts said.

With more children becoming obese, this could contribute to earlier breast cancer.

But these possible factors do not explain why someone as young as Taylor, who is not overweight and did not hit puberty early got breast cancer in her teens.

Likewise, Hannah's case is also perplexing.

"I was just like ... how could this happen?" Hannah said in June. "I'm 10. So, I was really shocked." Her family has no history of breast cancer.

The girl who played third base in her softball team and loved reruns of Hannah Montana learned she had a rare breast cancer that appears in less than 1 percent of total cases called secretory carcinoma. After a mastectomy in May and chemotherapy this summer, the sixth-grader went back to school.

This month, she underwent lymph node surgery, because doctors want to make sure that the cancer has not spread, said her father, Jeremy Auslam.

"We explain everything to her. She doesn't want to go through everything, but she does what she needs to do. She is involved," he said, about Hannah's role in medical decisions. The family maintains a blog chronicling Hannah's treatment.

Despite cases like Hannah's and Taylor's, the probability of a young girl getting breast cancer is slim, doctors said.

Oncologists Weiss and Litton discourage routine mammograms for younger women, because their breast tissues are dense, making it harder to detect abnormalities and resulting in unnecessary biopsies. Mammograms are recommended for women in their 40s, unless there is a family history.

Weiss, who wrote a book, "Taking Care of Your Girls," about breast health, said teens and young women should be aware, but not panic-stricken.

"It's important to start conversations about breast health early," Weiss said. The ages between 8-18 are critical because "that's when they're constructing their breast tissue. That's when what they eat, drink, medicines they take and personal products they use, how they use their body becomes the building blocks in the construction project as they lay down the foundation of their future breast health."