A prominent cancer organization is for the first time recommending Americans initiate colorectal cancer screening at age 45 instead of waiting until age 50, a threshold long endorsed by many other medical groups. The new guidelines, from the American Cancer Society, would extend routine colorectal cancer screening to an additional 22 million American adults between the ages of 45 and 49 and send a clear message that colorectal cancer, which has been rising in young adults, is no longer just a disease of older people.

Dr. Thomas Weber, who is the co-chairman of an early-age onset colon cancer task group for the National Colorectal Cancer Roundtable but who was not involved in writing the new recommendation, called lowering the age for first screening “a game changer” that could save thousands of lives.

“This is a very, very big deal,” said Dr. Weber, who is director of surgical oncology for the northwest region of Northwell Health. “Solid epidemiological data from our national cancer registries documents a dramatic increase in the incidence of colon and especially rectal cancer among individuals under the age of 50, and the vast majority of those cases are in the 40- to 49-year-old age bracket.”

The guidelines, released Wednesday, were published in CA: A Cancer Journal for Clinicians, a peer-reviewed journal of the American Cancer Society. They do not call for everyone to have a colonoscopy, the gold standard of colon cancer screening. Rather, to maximize the chances that people will take up screening, the cancer society wants physicians to offer patients a choice of six different screening options, from the most invasive procedure, colonoscopy, which can be done every 10 years, to lab tests done on stool samples that can be collected at home, which must be repeated more often and followed by colonoscopy if results are positive.