There's no reason to not do a colonoscopy on a 16 year old. It's up to the doc's discretion, so I assume he's the one making the decison that she may not need it for some reason. But if the symptoms warrant a colonoscopy, then you may need to consult with another doctor.
Have they seen a pediatric gastoenterologist. My son is scheduled for his 3rd colonoscopy and he is only 7. If you feel they need a colonoscopy (symptoms) I strongly suggest a second opinion. My son is being monitored for Gardner Syndrom/ FAP. In certain situations a colonoscopy is warranted, because in some situations a diagnosis may be too late in coming. Granted not knowing is alot less stressful in certain situations, but when you weigh the possible outcomes if we were not told, I would rather deal with the stress now.
They did one on my son when they misdiagnosed him with Chrone's and he had cancer instead, he was 15 yrs old. Just depends on the doctor.