My Mom had squamous cell carcinoma on her right calf when she was 95 and after enduring the radiation, she swore she would never have any more treatment that affected her quality of life. She only consented to non invasive testing for the remainder of her life. She lived to celebrate her 101st birthday with us. A month later she suffered a serious fall that required hospitalization and had to go into a rehab facility. She had a stroke a month later and passed away peacefully in her sleep a few days later.
The question is not whether to have a colonoscopy in your late 90's but whether you would consent to any aggressive treatment based on the results? If not, then the colonoscopy question becomes a moot point. If you refuse the colonoscopy, what is your doctor suggesting as the next course of action?