"Colorectal cancer had been thought a success story", as increased screening tamped down overall rates, says Siegel. The cancer society will look at whether that should change, Siegel said.
Rectal cancer rates in adults who are 55 and older have generally been declining for at least 40 years.
They are also at twice the risk of colon tumours, which start growing lower down.
The study found that colon cancer incidence rates increased by one to two percent per year from the mid-1980s through 2013 in adults ages 20 to 39.
The study was published today in the Journal of the National Cancer Institute. They used what's called "age-period-cohort modeling", a tool created to disentangle factors that influence all age groups, such as changes in medical practice and procedures, from factors that vary by generation, such as changes in behavior that may affect health.
From the mid-1980s through 2013, however, the rates of colon cancer among adults ages 55 and up declined, according to the study.
Both colon and rectal cancer incidence rates in 50 to 54 year olds were half those of 55 to 59 year olds in the early 1990s. For adults between 30 and 39, there was a similar rise from 1980 to 2013.
Siegel said she was surprised and concerned by the results.
For rectal cancer, the changes in the rates were more pronounced, the study said.
"Trends in young people are a bellwether for the future disease burden", said Siegel.
"We should look at whether starting screening at age 40 would make a difference", added Dr Kisiel, who recently conducted a clinical trial of Cologuard, a DNA test for colon cancer.
Many young patients have no obvious risks, Weber said, so "we suspect there may be additional factors at play". But what's more startling is that this isn't a surprise to anyone within our community - the data around this issue has been here since 2012, when we brought in experts from across the country for the nation's first young-onset symposium and released the breakthrough white paper highlighting this disturbing trend.
Although no definitive reason for the increased cancer risk for millennials was given, study leader Rebecca Siegel of the American Cancer Society said changes in diet, a sedentary lifestyle, being overweight, and low fiber consumption may be causing the rapid increase.
In addition, the authors suggest that the age to initiate screening people at average risk may need to be reconsidered.
The data showed a surprising trend: Despite a decrease in colorectal cancer rate for older adults, young adults' rates of colorectal cancers have risen to where they now have the same odds of being diagnosed with colorectal cancer as someone the same age, but born in the 1890s. But among younger adults, rectal cancer rates have been increasing even faster than colon cancer rates.