“I still remember getting the phone call from the doctor who did the procedure, who was completely shocked, and said ‘I would never have guessed that a 22-year-old would have had cancer,’” said Ms. Freiborg, who was treated with surgery and chemotherapy and is now cancer-free.

Most colorectal cancers are considered a disease of aging, so any increase in young adults, especially when rates of the disease are on the wane in older people, is both baffling and worrisome, experts say.

Colorectal cancer rates have declined over all in recent years thanks to widespread use of screening tests like colonoscopies, which can detect precancerous polyps that can be removed before cancer develops. These screening tests have not been considered practical for a younger population, and while other less invasive screening tests exist, doctors are hoping improved methods that will be easier to administer will be developed.

Experts also attribute lowering cancer rates to changes in risk factors, particularly lifestyle changes like smoking cessation and healthier diets. Diets that include more fruits, vegetables and fiber and less red and processed meat are linked to lower colorectal cancer risk.

Obesity and sedentary lifestyles are also associated with colorectal cancer, as are heavy alcohol use and chronic conditions like inflammatory bowel disease and Type 2 diabetes, all of which are on the rise. But experts are not entirely convinced these are the only reasons colorectal cancer is increasing among young people. While rates of cancers tied to human papillomavirus, or HPV, have been rising in recent years, that virus causes mainly cancers of the cervix, anus or the back of the throat, and only a small number of cases of rectal cancer.

“The honest truth is nobody knows 100 percent why there is an increase,” said Dr. Mohamed E. Salem, an assistant professor at Lombardi Comprehensive Cancer Center at Georgetown University. He said that he is older than about 60 percent of his patients — and he is 42. “It’s hard to blame it on obesity alone. We suspect there is also something else going on.”

Dr. Jason A. Zell, an associate professor of oncology at the University of California, Irvine, who did a study in 2014 that found increasing rates of colorectal cancer among adults ages 20 to 39 in California, said the big challenge is how to shape health policy. “By now we know the rates are going up, it’s been reported several times,” Dr. Zell said. “Now the question is, what do we do?”