How rare would it be that a person in his 20's would have colon cancer?

Extremely rare: This is quite uncommon, and usually only seen in people with an extensive family history of the disease, polyposis coli (a disease with thousands of polyps) or ulcerative colitis (an inflammatory disease of the colon known to [over many years] to cause cancer). You should be tested for genetic markers for colon cancer and other cancers to determine best treatment for you, and screening for family.

Quite rare; not zero: Colorectal cancers in young adults are most often due to a genetic problem like hnpcc (hereditary nonpolyposis colorectal cancer) or fap (familial polyposis coli). Cr cancers can also be seen in young adults with inflammatory bowel disease, although it is rare. Any young adult with symptoms that don't respond to treatment should seek further evaluation.