30yrmale. Burning anus after stool&stays till evening. Leg thigh pain. No blood in stools. Doctors wont do colonoscopy saying cant be cancer at this age?

Get second opinion. Although the risk of colorectal cancer increases with age and is rare in at age 30, it is possible. It sounds like the source of your pain and discomfort has not been identified. If your condition were properly diagnosed and treated, you would be less likely to want a colonscopy, which probably is not indicated.