Colon cancer is typically a condition mainly associated with older adults, but that is starting to change. Although most colon cancer still happens in older adults, early-onset colon cancer is more often affecting younger populations around the world. This shift has left many young adults shocked by a diagnosis they never expected, often after symptoms were overlooked or dismissed.

Early-onset colon cancer, also called young-onset colon cancer, is defined as colon cancer diagnosed before age 50.

Colon cancer usually begins as small clumps of cells called polyps that form in the section of the large intestine called the colon. Rectal cancer starts as a growth of cells in the last several inches of the large intestine, called the rectum. Cancer inside the rectum and cancer inside the colon are often referred to together as colorectal cancer.

In about 20% of people with early-onset colon cancer, a genetic condition is the underlying cause. However, most people diagnosed with early-onset colon cancer have no such condition.

Certain risk factors are common across all age groups, but early-onset colon cancer appears to have its own distinct risk profile, especially among people without genetic risk factors. These unique factors include:

• Gut microbiome changes. Studies show that people with colorectal cancer often have less variety in their gut bacteria compared with that of healthy people. Certain bacteria are found more often in people with cancer. These bacteria can make substances that damage DNA and cause inflammation.
• Antibiotic use. Using antibiotics, especially early in life, may disrupt the gut microbiome, increasing long-term cancer risk.
• Western diet early in life. High intake of sugary drinks and processed foods in adolescence and young adulthood has been linked to greater early-onset colon cancer risk.
• Sedentary behavior in youth. Sitting for long hours during adolescence and early adulthood — watching TV or working at a desk, for example — is strongly linked to early-onset colon cancer and rectal cancer.
• Certain genetic variants combined with environmental factors. Some people may have small changes in their genes that don't cause problems on their own. But when combined with factors such as a high-fat diet, these gene changes might lead to colon cancer. For example, one gene called HNF1A may only cause cancer if someone also eats a lot of foods that are not healthy.
• Lack of screening. Most people don't get checked for colon cancer until they turn 45, which is when doctors recommend starting screening for those at average risk. This means younger people usually aren't screened unless they have symptoms. But signs such as stomach pain or bleeding are often mistaken for something minor, so testing might be delayed. When cancer isn't found early, it can be harder to treat.

The average risk of getting colon cancer when you're younger is still low but rising. In the early 2000s, about 5% to 7% of colon cancer diagnoses were considered early onset. Now, around 10% of colon cancer diagnoses happen in people younger than 50. Most of these diagnoses happen between the ages of 40 and 49. 

Here's the risk of being diagnosed with colon cancer by decade: 

• 20s: About 2.3 people in 100,000.
• 30s: About 6.4 people in 100,000.
• 40s: About 19.2 people in 100,000.

Keep in mind that having a first-degree relative with colon cancer increases your risk by more than four times. A first-degree relative is a parent, sibling or child. Some inherited conditions, such as  Lynch syndrome or familial adenomatous polyposis (FAP), can dramatically increase your lifetime risk of colorectal cancer. For people with Lynch syndrome, the risk can be as high as 80%, and for those with FAP, the risk can approach 100% without preventive surgery.

Unlike older adults who may be diagnosed through routine screening, most younger people notice symptoms. These symptoms are often mistaken for less serious conditions, such as stomach upset or hemorrhoids. Because of this, diagnosis is sometimes delayed. It's important to know the signs and talk with a healthcare professional if they don't go away. 

Common symptoms to look for include:

• Rectal bleeding. Almost half of young people with colon cancer have rectal bleeding. Blood in your stool or on toilet paper may be a sign.
• Stomach pain or cramping that doesn't go away.
• Changes in bowel habits, such as new or lasting constipation or diarrhea.
• Feeling very tired or weak, which may be caused by low iron.
• Weight loss without trying.
• Bloating or feeling full soon after eating.
• Nausea or vomiting.

These symptoms are generally consistent for many people with early-onset colon cancer.

The prognosis of early-onset colon cancer can vary depending on a few factors, including: 

• Stage at diagnosis. Young adults diagnosed in earlier stages of colon cancer have a much better prognosis compared with those diagnosed at stages 3 or 4. Early-onset colon cancer is often diagnosed at more-advanced stages. This is often due to delayed recognition of symptoms, which can worsen prognosis.
• Type of cancer cells. Colon cancer in younger people often is more aggressive. Despite this fact, many young adults usually survive just as long, or even longer, than older adults when they are treated at the same stage.
• Treatment response. Younger people often receive more-aggressive treatments, including surgery and chemotherapy, and typically tolerate these therapies better than do older adults.
• Genetic differences. Unique genetic profiles in young adults may influence cancer cell behavior and treatment response. However, this is still being studied.

The survival rate for early-onset colon cancer varies significantly by stage at diagnosis, just like in older adults. However, young adults tend to have better survival outcomes when matched with older adults by stage, despite often being diagnosed later. This is possibly due to resilience and better treatment tolerance.