Risk factors at any age include obesity, lack of physical activity, a diet high in red or processed meat and low in fruits and vegetables, smoking, heavy alcohol use, having inflammatory bowel disease or a family history of colorectal cancer.

No one knows what’s causing the rise in cases in younger adults. But Georgetown’s Marshall said many young patients lack common risk factors. He wonders if changes in young people’s gut bacteria — the microbiome — might play a role.

Still no real answer to this question, then. The article lists risk factors but the younger people don't necessarily have them. Increasing exercise if you're sedentary might help. Speculation on my part feels wrong when people who have studied this for years are still speculating, themselves.