CHICAGO (CBS) — Colon cancer is no longer a disease just for seniors.

“Someone born in 1990 would now have twice the risk of colon cancer and four times the risk of rectal cancer at the same age [as they would] if they were born in 1950,” said Dr. Scott Strong, chief of gastrointestinal surgery for Northwestern Medicine. “What we’re seeing is an increase in the number of individuals under the age of 50 being diagnosed with colon and rectal cancer relative to historic times.”

Strong said the trend is a disturbing one.

“I was involved in the treatment of a woman, just last week who was 23 years old with no family history and no risk factors,” he said.

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While doctors haven’t been able to pinpoint exactly why younger people are being diagnosed, Strong said environmental factors and lifestyle could be contributing factors.

“There are risk factors we can’t control and those we can. Those things that you can control are being overweight, or obese and physical inactivity. Both of things put you in increased risk for colon cancer. There are certain kinds of diets to avoid. Those high in red meats, processed meat; and low in vegatables, fruits and whole grain fibers. Smoking is also linked to colorectal cancer,” he said.

While screening recommendations remain at age 50, Strong said it never hurts to start early. Most young adults are tested only after symptoms develop, but by that time the disease is more advanced.

“Unless they have family history or any other reason for screening, health officials aren’t going to recommend it, and young people aren’t going to seek it out, so you’re really left with just listening to your body and responding to symptoms,” Strong said.

Most of the nation’s 135,000 annual cases and 50,000 deaths related to colon and rectal cancer still occur among people over age 55, but the share of cases involving younger adults has risen to 29% for rectal cancer and 17% for colon cancer, the study showed. About 11,000 people in their 40s and 4,000 under 40 were diagnosed in 2013.

Among possible symptoms: bloody stool, cramps, unexplained weight loss and changes in bowel habits that persist more than a few days.

Experts stressed that the absolute number of colo-rectal cancer in young people is still very low. Americans over age 50 are actually 16 times more likely to get that cancer. Doctors say a a high-fiber, low-fat diet can reduce the risk of developing colorectal cancer.

March is Colon Cancer Awareness Month.