Colorectal cancer is often considered a disease of aging, with up to 90 percent of cases diagnosed in people over 50. Now scientists report a sharp rise in cancers of the colon and rectum in adults as young as their 20s and 30s—and no one is quite sure why, according to a new study from the American Cancer Society, The New York Times reports

Since 1950, colorectal cancer rates appear to have increased for every generation in America. Today findings show that folks in their mid-20s experience double the risk of colon cancer and quadruple the risk of rectal cancer when compared with individuals of their parents’ generation.

Cancer experts also noted that colorectal cancer rates had decreased steadily for people born between 1890 and 1950. But since the 1970s, incidence rates of rectal cancer jumped by 3.2 percent a year among adults in their 20s. Meanwhile, colon cancer rates among adults ages 20 to 39 rose by 1 percent to 2.4 percent a year during the past 30 years. 

Researchers also warned that young people with colorectal cancer might face the added risk of getting diagnosed later in the course of the disease, when the illness is less treatable. Currently, routine colonoscopies, which screen for such cancers , aren’t recommended until adults are 50 or older. What’s more, when patients are so young, even with the presence of symptoms, scientists said doctors don’t typically consider a colorectal cancer diagnosis right away.

Obesity and sedentary lifestyles are associated with an increased risk for colorectal cancer, as are heavy alcohol use and chronic conditions, such as inflammatory bowel disease and type 2 diabetes—illnesses that are on the rise among America’s young people. But experts aren’t entirely convinced that these health problems are the only reasons for the uptick in colorectal cancers among young people.

“The honest truth is nobody knows 100 percent why there is an increase,” said Mohamed E. Salem, MD, PhD, assistant professor at Lombardi Comprehensive Cancer Center at Georgetown University. “It’s hard to blame it on obesity alone. We suspect there is also something else going on.” 

The risk for colon cancer is also higher among African Americans. The American College of Gastroenterology recommends that Black people start screening at age 45, not 50. Additionally, doctors explain that although warning signs of colon cancer are vague, symptoms of the illness, such as diarrhea, constipation, cramping, abdominal pain, weight loss and bloody stool, shouldn’t be ignored.

Click here to learn more about colon cancer and how to prevent it.