Much has been written — and TikTokked — about how well millennials are aging, with the consensus being that the generation looks younger and fresher in their 30s and 40s than previous generations did at the same age.

But the youth-boosting effects of sunscreen and skincare that are so apparent on the outside may not be offering quite the same boon to the inside.

A new study suggests that people born in the ’90s are biologically aging faster than Gen X did — and it’s likely at least partly to blame for surging cancer rates.

One person, different ages

In the study, researchers compared biological age (how old the body appears to be in medical testing) and chronological age (how old the body actually is)

Compared to those born in 1965–1969 (Gen X), people born in 1990–1999 (younger millennials and older Gen Z — the cutoff is around 1996) had a 92% higher PhenoAge-defined age gap. In other words, people born in the ’90s showed older biological age than people in the older generation when at the same chronological age.

Male participants consistently showed bigger biological/chronological age gaps than female participants.

The worst news? The larger the gap, they found, the higher the risk of tumorous cancers, like lung, gastrointestinal, colorectal and uterine.

More young people are getting cancer

Between 1990 and 2019, the number of people under 50 diagnosed with cancer increased by 24%.

Certain cancers are becoming more prevalent in younger generations, including colorectal and uterine cancer.

People born in the 1990s are more than four times more likely to get early-onset colorectal cancer compared to those born in the 1960s. In the US, the proportion of colorectal cancers diagnosed before age 55 increased from 11% in 1995 to 20% in 2019.

Those born around 1985 face twice the risk of uterine cancer compared to those born before 1950.

Why is this happening? Researchers believe there might be a connection to other trends, like puberty starting earlier, and earlier onset of obesity, diabetes and stroke. These all point to faster aging, and act as risk factors for cancer, they said.

Parts of the body could age faster than the whole

An organ system’s age could look different than the whole, researchers said. Different organ systems can also age differently within the same body.

Researchers found that an immune system that appears older than its actual age was associated with early-onset lung cancer. Similarly, fat tissue that appears older than its chronological age was associated with early-onset colorectal cancer.

Until now, there hasn’t been much research about how aging varies by generation, which is clearly playing a role in increased early onset cancers, researchers said.

“Right now, we don’t have a definitive answer to what’s driving the rise of early-onset cancers around the world,” said David Scott, director of Cancer Grand Challenges, the organization that helped fund the research.

“But studies like this are helping us piece together the bigger picture, showing that cancer may be influenced not just by changes inside individual cells, but by wider changes happening across the body as a whole.”

In the future, researchers hope to understand how modern environments drive cancer risk, to eventually help prevent it and better treat it, said author Yin Cao, a molecular epidemiologist and an associate professor of surgery and of medicine at WashU Medicine.

Cao and her colleagues will examine how environmental, lifestyle and societal changes leave lasting biological imprints, including accelerated aging.