If you don’t have any symptoms and are at average risk, colorectal cancer screening should start at age 45. A colonoscopy is the main way to check for diseases of the colon, such as colitis or cancer, and to remove colon polyps. A polyp is a mushroom-like growth on the inside wall of the colon or rectum. Polyps grow slowly over many years. Some polyps become cancerous, others do not.