You do need to be aware of your
families health history(Has anyone
in your family had any of those diseases I mentioned or
colon cancer?)
You should go to a doctor regardless, again because if you have been having blood in your bowel movements for years, you run the risk of being anemic. Meaning your body is continuously losing blood.
Your doctor will check you for probable causes like hemorrhoids,etc. Most common reasons for blood at your age.