Sure, you can get a colonoscopy whether or not you are a virgin. They aren't doing anything to your vagina, it's your anus that is the business end with a colonoscopy. I don't know how much they cost, but if it is a lot, I'd just try to get some insurance instead. The only thing is, they might not book you for one if you're only 21, since colon cancer is not a young person's disease, unless you have a recommendation from a doctor.