I don't know anything need advice

I'm 18m and suffer from serious bowl problems I often get diarrhea for no reason and I'm prone to hemroids. I'm scheduled for an appointment soon and don't know what to expect. I've done some research on stuff and had a laugh at PEG aka polyethylene glycol but even through my immature humor I'm a bit nervous.

What should I expect and how long could this take. And what differences are there between a young adult and someone in their 50s getting a colonoscopy.