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.
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.
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.