If 'pencil' stool and 'narrow' stool are signs of colon cancer but exactly how thin of the stool consider as pencil stool or narrow ?

Only a descriptive: Term used to describe major changes in ones bowel habits. Size or measurement would not be used. Age 24 would be extraordinarily rare to get colon cancer. Mainly bowel function is based on your diet, and likely need higher fiber diet with fruits and vegetables, avoid junk foods, add metamucil, drink plenty water. See a doctor for an examination.

Any change.: Any change in stool caliber warrants investigation. If one's stool has changed in size (narrow), to any degree, a colonoscopy is warranted. This could be an early sign of colon caner.