24 y/o male. occasional flat or thin stools. but usually normal. no blood (that i know of). no real abd pain. occasional constipation. colon cancer?

See doctor: At your age probably not but any change in bowel caliber warrants consideration of a colonoscopy. You should really discuss in more detail with your doctor.

Not likely: Try taking a bulk laxative like metamucil, a couple of table spoonfuls each day for a couple of weeks to see if the symptoms resolve. If that does not help, it would be time to see a doctor.