The organization, which announced the change in its guidelines Wednesday, said extensive analysis showed that lowering the starting age for screening would save lives. The recommendations apply to adults who are at average risk of the disease; this includes most people in the United States.

George Chang, professor of surgical oncology and health services research at MD Anderson Cancer Center in Houston, welcomed the change. “We see a lot of younger patients,” he said. The reason the rates are rising “is the million-dollar question,” he said, noting that factors may include increasing rates of obesity, a lack of exercise and the consumption of processed foods.

Over the past two decades, the incidence of colorectal cancer has declined steadily among people older than 54, largely because screening has increased, allowing the identification and removal of polyps that can become malignant. But since 1994, there has been a 51 percent increase in the rate of the disease among those younger than 50, and the death rate also has begun to rise.

A recent analysis found that adults born around 1990 have twice the risk of colon cancer and four times the risk of rectal cancer, compared with adults born around 1950, the cancer group said. As they age, younger adults will continue to have an elevated risk, compared with previous generations, studies suggest.

Stefanie Joho, who was diagnosed with colon cancer at 22, has been in remission since February 2016, thanks to an immunotherapy drug that targets a genetic mutation.

When the cancer society started to update its recommendations, it initially focused on racial groups that have higher death rates from the disease, primarily African Americans. But when the data pointed to a “persistent increase” among younger people across demographic groups, the society decided to recommend younger screening ages for all adults, said Richard Wender, chief cancer control officer for the group.

Experts said many primary-care physicians are likely to adopt the new recommendations and begin referring younger patients for screening. But not everyone agrees with a starting age of 45. The U.S. Preventive Services Task Force, an independent panel that evaluates screening tests, decided a few years ago not to lower its recommended age from 50. It concluded that the data was mixed and that a younger starting age would provide only a “modest” benefit.

The cancer society, in coming to a different decision, extended the analyses used by the task force and incorporated recent data on the rising incidence of the disease among younger people. That produced a result that showed the lifesaving potential of earlier screening, the group said.

People with a family history of colorectal cancer are already advised to start getting screening tests before age 50.

Colorectal cancer is the fourth-most-common cancer diagnosed among adults in the United States. The cancer society estimates that in 2018, more than 97,000 Americans will be diagnosed with colon cancer and more than 43,000 will be diagnosed with rectal cancer. About 50,000 people are expected to die of colorectal cancer this year.

The colon is also known as the large intestine; the rectum is the part of the intestine that descends from the colon to the anus. Colorectal cancer has not been linked to the human papillomavirus (HPV), which can cause anal cancer, as well as cervical, throat, penile and other types of cancer. A vaccine can prevent transmission of HPV.

For colorectal cancer, the American Cancer Society didn’t push one screening option over another but listed various options: High-sensitivity stool tests, designed to detect blood in feces, which need to be administered every year; a DNA stool test, sold under the brand name Cologuard, every three years; a colonoscopy, every 10 years; or a virtual colonoscopy or flexible sigmoidoscopy, every five years. Patients who test positive for signs of cancer on a non-colonoscopy screening method should follow up with a colonoscopy quickly, the group said.

The rate of colorectal cancer among the 50-to-54 age group, even with a decline over the past several years, remains higher than among those who are 45 to 49; that partly reflects the start of screening at 50, which leads to detection of the disease and allows people in the older group to be identified as having colorectal cancer. The cancer organization believes that the underlying risk of the younger group is probably close to that of the older group.

Chang, of MD Anderson, said he thought that 45 years old “makes sense” for the start of screening. But he noted that younger people are also at increased risk. A 2014 study, of which he was the senior author, showed that by 2030, based on current trends, rates for colon and rectal cancers will increase by 90 percent and 124 percent, respectively, for patients 20 to 34 years old.

Andrea Cercek, an oncologist at Memorial Sloan Kettering Cancer Center in New York, said the updated guidelines may “capture a population at risk.” But she also pointed out that colorectal cancer rates are rising rapidly in adults younger than 45 and urged people to seek medical attention promptly if they experience “persistent and concerning gastrointestinal symptoms.” Signs of colorectal cancer may include a change in bowel habits, such as diarrhea or constipation, that lasts more than a few days; rectal bleeding; and cramping or abdominal pain.