Colon cancer rates among young adults are on the rise in the United States, Canada, and seven other wealthy nations, even though rates among older adults are down or stable, a new study finds.

The researchers analyzed data for 36 countries and found that over the past 10 years, colon cancer rates among people under age 50 were stable in 14 countries, fell in three (Italy, Austria and Lithuania), and rose in 19 countries.

In several of those nations, a rise in cases affecting young adults (early-onset colon cancer) contrasted with rates in people aged 50 and older, which either dropped (Australia, New Zealand, Canada, Germany, and United States) or were stable (Denmark, Slovenia, Sweden, and United Kingdom).

In all but one of the 19 nations, the uptick in early-onset colon cancer began in the mid-1990s, the investigators found.

In three countries (Cyprus, the Netherlands and Norway) where rates rose in both groups, the increase in young adults was twice that in older adults. It also began in the mid-1990s.

The largest rise in early-onset colon cancer — 4.2% a year — was in South Korea, which has the highest rate of all countries studied, according to American Cancer Society research published Sept. 5 in the journal Gut.

The findings suggest that changing exposures in early life are increasing the risk in some countries, and there is an urgent need to learn more about early-onset colon cancer, according to study leader Rebecca Siegel. She's the cancer society's scientific director of surveillance research.

"Although the absolute risk of [colon cancer] in adults younger than 50 years is low relative to older adults, disease trends in young age groups are a key indicator of recent changes in risk factor exposures and often foreshadow the future cancer burden," Siegel said in a cancer society news release.

Worldwide, colon cancer is the third most commonly diagnosed cancer, with an estimated 1.8 million new cases in 2018.