Colon cancer runs rampant in my family. My dad was a stage 3 survivor (it's now been 12+ years), however his younger sister (my aunt) just died about 6 weeks ago after an 18 month battle. She was a former nurse, and had basically missed getting one of her regular colonoscopies. At that time, she was supposed to go every 5 years and I think she went 10 - too long for her age. She ended up with a blockage, which by that time meant things were realistically too late. She was only 64 and in otherwise excellent health. A second, younger aunt on that side suffers from diverticulitis and has had bowel surgery.
The cancer gene also runs on my moms side; an aunt there had both colon and breast but survived both. So, I'm pretty much screwed. As a result of those risk factors and various symptoms, I've personally had two colonoscopies (one just a month ago).
I'm close to 40, and I had a 2mm polyp removed. There's a high chance that such a polyp would have developed into colon cancer within a few years. I'm now on the 3-year plan for colonoscopy checks. The good news is, in general, colon cancer is one of the most successfully treated diseases if caught early enough. Gastroenterologists will tell you that there's no excuse to die of colon cancer if you get regularly checked.
What I would tell people is to get checked by age 40. If you have any symptoms don't hesitate to call a gastro and get in for a check. For most people, it is hemorrhoids or something else - but I've got a friend (39 years old) who was diagnosed with stage 4 colon cancer - the point is this thing is hitting people in their 30s, so it needs to be taken seriously if you're symptomatic.
On the topic of prep, it's not that bad. It's basically 24 hours of suckage because you're on liquid diet for a day, and taking the prep that evening (typically split-prep, some before bed, some in the middle of the night). Try to schedule your colonoscopy for the first thing in the morning so that you don't have to go more than a day without solid food.
(edit: spelling correction)