At what point do i need to worry about colon cancer? i've had pencil thin stool for a week now and can't completely empty my bowels.

Unlikely at your age: It would be very unlikely for someone of your age to have colon cancer, unless there's a strong family history that would suggest a genetic predisposition for such. However, with these pencil-thin stools, you may have an infection or inflammation within the colon which needs identification and treatment. A CT scan and colonoscopy would be in order to investigate this.