How likely is it for a 21 year old to have colon cancer with no family history and no other bowel diseases?

Very low chance: It is rare for 21-y-o to have a colon cancer, even more so without family history.

Cancer Fear: As Dr. Park mentioned, the probability of you getting cancer is very low. Create a relationship w/a family doctor if you have not done so and be followed by her/him as suggested. If the concern continues to bother you consider consulting a clinical psychologist. Cancer phobia is an unfortunately common one just as HIV was in the 80s. Peace and good health.