Ryan Decembrino was only 29 when doctors found a tumor in his colon. A colonoscopy two years earlier had uncovered a dozen polyps, but because cancer didn’t run in his family, he was told to wait three years for a follow-up. That advice nearly cost him his life.

“If I would have waited until my three-year follow-up, I wouldn’t be here today,” Decembrino told Science News. Now cancer-free after surgery and chemotherapy, he has become an advocate. “We have to get the word out,” he says. “Cancer can happen to anyone.”

Decembrino’s story is part of a troubling trend. Typically, we consider cancer to be an older person’s disease — 88 percent of U.S. cases occur in people over 50. But since the 1990s, rates of early-onset cancer, defined as cases diagnosed before age 50, have been climbing worldwide.

A Growing Epidemic Among the Young

The uptick in early-onset cancers started to garner public attention in 2020 when the actor Chadwick Boseman, the star of the Black Panther film series, died from colorectal cancer at the age of 43. 

On the big screen, Boseman was handsome, muscled, and by all appearances the epitome of well-being. He was the last person many people expected to be diagnosed with, let alone die from, cancer.

But relative youth is no longer the forcefield against colorectal cancer that it used to be. Early-onset colorectal cancer “is becoming the leading cause of cancer deaths among young adults in the United States,” said Yin Cao of the Washington University Siteman Cancer Center in St. Louis. 

The numbers are startling. Between 1990 and 2019, global incidence of early-onset cancer rose by 79 percent, while deaths jumped 28 percent. A study in Lancet Public Health found that people born after 1990 face a steadily rising risk for at least 17 different cancers, especially of the small intestine and pancreas.

“People born after the 1980s are four times more likely to be diagnosed with rectal cancer than those born around 1950,” Hyuna Sung, a cancer epidemiologist at the American Cancer Society, told Science News.

A Worrying Reality

If current trends continue, by 2030 early-onset cancers will rise another 31 percent worldwide, with deaths up 21 percent. Millennials and Gen Z could carry disproportionate cancer risks for the rest of their lives.

“This is serious and worrisome,” says Shari Goldfarb, breast oncologist and Director of Memorial Sloan Kettering’s Young Women With Breast Cancer program.

“This is not a blip,” explains Andrea Cercek, a gastrointestinal oncologist at the Memorial Sloan Kettering Cancer Center. “The more data we gather, the clearer this becomes.”

Doctors on the frontlines see it happening. “Why are we seeing more and more younger people in our clinics, and why are they … presenting [cancers] at more advanced stages?” asks oncologist Alok Khorana of the Cleveland Clinic.

Better screening can’t fully explain the rise. “It can’t account for this kind of broad, across-age effect,” Sung notes. Instead, researchers suspect that a cocktail of modern environmental exposures and lifestyle shifts may be at play.

What Could Be Driving Early-Onset Cancer?

Scientists are following several leads, but no single culprit explains the surge. Instead, the evidence so far points to a mix of factors that together may nudge the incidence of early-onset cancer.

Diet and obesity: The spread of the so-called Western diet — high in processed foods, sugar, and red meat — is one of the strongest suspects. “The more components of the Western diet, the greater the risk of early onset colorectal cancer,” says Khorana. But studies show the link is complex. Obesity may also be part of the picture, yet not the whole story.

Chemicals and plastics: Microplastics and “forever chemicals” known as PFAS are under scrutiny. These pollutants have been detected in human tissues and, in some cases, inside tumors. “The evidence that microplastics cause early onset cancers is weak but increasing,” says Frank Frizelle, a colorectal surgeon at the University of Otago. While causation hasn’t been proven, the timeline of rising plastic use eerily matches the increase in young cancer cases.

Gut microbes: Colorectal cancer, which now ranks as the second leading cause of cancer death in U.S. women under 50 and the first in men, may be tied to shifts in gut bacteria. A 2024 study in Nature linked early exposure to colibactin, a toxin produced by certain E. coli, to genetic mutations commonly observed in young colorectal cancer patients. “These mutation patterns are a kind of historical record in the genome,” says study leader Ludmil Alexandrov of UC San Diego, “and they point to early life exposure to colibactin as a driving force.”

Still, colibactin likely explains only a fraction of cases. “It makes you think that it’s probably a combination of a number of different factors,” says Sonia Kupfer, a gastroenterologist at the University of Chicago.

The Challenge of Early-Onset Cancers

Some scientists think what we’re watching with colorectal and other cancers isn’t just random bad luck but a classic birth cohort effect. For most of the 20th century, colorectal cancer showed up mainly in people hitting their 60s or 70s. But something shifted for those born after the 1950s. The environments they grew up in — the food, the chemicals, the everyday habits — seem to have sped up how quickly the disease takes root.

In other words, an entire package of shared exposures has opened a new biological window. That window is letting cancer creep into younger bodies decades before it used to.

Then, even when young people show symptoms, their cancers often get overlooked.

Samantha-Rose Evans, a 26-year-old administrator in England, was told by doctors not to worry about her irregular periods. Her fiancé urged her to push for answers — and that persistence led to a diagnosis of endometrial cancer. “My previous doctors had told me I shouldn’t be concerned,” she says.

Misdiagnosis is common. A U.S. survey of nearly 900 people with early-onset colorectal cancer found that 54 percent were first told they had something else, usually hemorrhoids. More than a third saw three or more doctors before learning the truth.

That delay can be deadly. Some scientists now believe early-onset tumors may grow faster than typical cancers, advancing in one to two years instead of the usual five to fifteen. If true, current screening schedules may be too slow to catch them in time.

Awareness, Advocacy, and What Comes Next

For now, the best advice is still the basics: get vaccinated against HPV and hepatitis B, eat well, exercise, and avoid tobacco and excess alcohol. But as Rosalind Holden, diagnosed with uterine cancer at 38, puts it: “If anything worries you, especially women’s health like heavy periods, get it checked.”

Advocacy campaigns are starting to make a difference. After actor Ryan Reynolds promoted colonoscopies, appointments jumped 36 percent in just three weeks, according to Zocdoc. But experts stress that awareness among doctors is equally important, although more and more doctors are waking up to the fact that they should also be on the lookout for cancer in younger patients as well.

Despite the grim statistics, there is one piece of good news: overall cancer mortality continues to decline, thanks to better treatments and earlier detection in older populations.

“Cancer sucks,” Decembrino says. “The only way I got through it was by having great people around me. I’m part of a buddy program now, but thankfully I haven’t needed to be called up yet.”

His message, and that of many other young survivors, is simple. Don’t assume you’re too young for cancer. And don’t wait to check.