When should someone see a gastroenterologist?

Depends... Most initial digestive problems can be addressed by your primary care doctor, but when further diagnostic procedures are needed, or when initial treatment attempts are unsuccessful, a gastroenterologist may be necessary. Also, everyone should see a G.I. doctor at age 50 or sometimes earlier, if there is a family history, for colon cancer screening.
It depends. A gastroenterologist performs colonoscopy and endoscopy procedures. Patients only consult with a gastroenterologist if they need to have a screening colonoscopy or if they are having gastrointestinal problems that require expertise beyond that of a primary care physician. Screening colonoscopies begin at age 50, or sooner if you have a family history of colon cancer.
Gastroenterologist. When one suffers from a gastro-intestinal disease including liver, gall bladder, pancreas, that cannot be managed by a primary care physician.
See yr primary first. I tend to refer to GI for a variety of reasons: if my list of possible diagnoses has not successfully resolved the problem, a procedure (like colonoscopy, ERCP, G-tube placement) is needed, a specific treatment protocol (like hepatitis C mngt & treatment) is needed, for example. It's important to see your primary clinician first in order to address the most common considerations. Hope that helps.