At what age would having blood in stool make a doctor start to worry about cancer? age 20 years? age 25 years?

ALL ages: Normally there is no human blood in human stool. Particularly some older tests may detect dietary blood from meat, etc . Bloody diarrhea suggests an invasive organism. Hemorrhoids can bleed - a common cause of bright red blood in stool. Black, tarry stools can result from bleeding in the stomach. Any type of blood in stool should lead you to consult a doctor; risk of cancer varies.

Any age: Blood in stool, at age, is abnormal and needs investigation. Depending on family history cancer may occur in the 20s.