From experience, if you are showing symptoms that could be colon cancer (or crohns/irritable bowel disease/ulcerative collitis), they'll send you for a colonoscopy. If you've have pretty severe IBS - colonoscopy to make sure its nothing else, since there is some crossover in the symptoms of IBS and crohns. Even if they are 99.9% sure that your symptoms are not crohns or colon cancer, they will send you for a colonoscopy.
That is likely how people in theres 30s got a colonoscopy. I had one when I was 21 for that exact reason. My GP was 99.9% sure that I did not have crohns, but he did not want to risk the chance that it was.
The colonoscopy itself is not particularly pleasant, and so I would not go for one unless there is a medical need to do so. But if you are showing symptoms, and you are concerned, yeah, you can try and ask for one, but I wouldn't do it for the sake of it.