What are the reasons to get a colonoscopy if still in 30's? and what all does a colonoscopy show ? is stomach pain, and different color stool usually figured out by endoscopy? thank you?

A colonoscopy is an exam used to detect changes or abnormalities in the large intestine and rectum. An upper endoscopy is a procedure used to visually examine your upper digestive system with a tiny camera on the end of a long, flexible tube. One is lower, the other is upper. Age has nothing to do with it. Let your gastroenterologist do their job. ref: mayoclinic.org

A colonoscopy is done regardless of age for diagnostic purposes and if there is pathology affecting the colon, a colonoscopy will help in diagnosing the respective condition by direct visualization and/or biopsies if needed.