Colorectal cancer can no longer be called an old person's disease. That is not my line, it is Ahmedin Jemal's, senior author on the American Cancer Society's new tally of cancer deaths in Americans under 50, and he did not choose the words casually. His team's research letter in JAMA, published in January 2026, tracked cancer deaths in that age group from 1990 through 2023. Colorectal cancer mortality, which had been climbing since 2005, rose about 1.1 percent a year, the only one of the five leading causes of early cancer death to rise at all, and it climbed from fifth place to first: ahead of breast cancer, which held second even as its own death rate fell; ahead of lung cancer, which dropped from first to fourth; ahead of leukemia, now fifth. "We weren't expecting colorectal cancer to rise to this level so quickly," Jemal said of that finding, "but now it is clear that this can no longer be called an old person's disease. We must double down on research to pinpoint what is driving this tsunami of cancer in generations born since 1950."
In adults 65 and older, colorectal cancer is doing what forty years of screening was built to do: incidence and deaths are both easing, in roughly the same range, a bit over two percent a year, according to the American Cancer Society's companion report, Colorectal Cancer Statistics, 2026, published in CA: A Cancer Journal for Clinicians this March. In adults under 50, incidence is climbing about three percent a year, and has been long enough that people born after 1950 now appear to carry a higher lifetime risk than the generations before them, a pattern epidemiologists call a birth-cohort effect, the same shift Jemal was describing above. Rectal cancer specifically is moving faster than colon cancer, up about one percent a year since 2018, and now accounts for close to a third of all colorectal diagnoses (32 percent), up from about a quarter in the mid-2000s (27 percent). Whatever is driving this, it is concentrating in the lower end of the bowel, in younger bodies, and it has been building since before most of today's patients were born.
Here is the number I want a reader who is not yet 50 to sit with. Screening for average-risk adults in the United States now starts at 45, lowered from 50 in 2021 after modeling data showed the earlier start caught more cancer for the added cost and risk. It was a real, defensible improvement. And yet, by the ACS's own figures, half of all colorectal cancers diagnosed before age 50 occur in people under 45, entirely outside that net. Only 37 percent of the newly eligible 45-to-49 group are even up to date on the screening they do qualify for. So the honest picture is not "we lowered the age and solved it." It is that we lowered the age and still miss half of them. Across early-onset colorectal cancer broadly, roughly 70 percent of cases have no family history of the disease at all, which means family history, the shortcut doctors most often use to decide who needs an early colonoscopy, fails to flag most of these patients regardless of their age.
What actually causes this, I cannot tell you, and neither can the people who study it for a living. Diet is the most established contributor and the least specific one: ultra-processed food, red meat, and inactivity are all associated with colorectal cancer generally, but researchers are candid that these exposures are common across the whole population and do not explain why the rise is concentrated specifically in people born after 1950. The gut microbiome is a livelier but thinner hypothesis, dysbiosis interacting with metabolic and immune pathways; a 2025 systematic scoping review of the evidence called the literature still too limited to draw conclusions from. The most concrete lead is a Nature paper from April 2025, led by the cancer geneticist Ludmil Alexandrov at UC San Diego, which found that colibactin, a DNA-damaging toxin made by certain gut bacteria, leaves a mutational signature 3.3 times as common in colorectal cancers diagnosed before 40 as in those diagnosed after 70, timed to have occurred within the first decade of life. Alexandrov's own words: acquire the wrong mutation by age ten, and you could be "decades ahead of schedule," developing at 40 what would otherwise have shown up at 60. He also said, in the same breath, that this supports a hypothesis and does not prove one.
That is where the honest version of this story has to end, not on reassurance and not on alarm. Something changed for people born in the second half of the last century; the data are clear on that much. Ranked by how well the evidence actually holds up, diet is the best established and the least specific to the young; the microbiome is a promising but still-thin thread; childhood exposure to colibactin is the newest and most specific lead, and still, by its own discoverer's account, only a hypothesis. A 2023 case-control study of more than 5,000 early-onset patients found that close to half, 49.3 percent, had their first identified warning sign, most often abdominal pain or rectal bleeding, within three months of diagnosis. A further 19.3 percent had a warning sign earlier still, three months to two years out, and for that group, the ones whose bleeding or pain got read as something else, the median wait before a diagnosis ran seven to twelve months, depending on which symptom it was. The remaining roughly third of patients had no red-flag sign the study could identify in that two-year window at all. Nothing here tells a reader under 45 what to expect. It only says two things plainly: a birthday under the screening line is not, on its own, proof that anyone is already looking for it, and a symptom that persists warrants medical attention regardless of which side of that line a person falls on.
