Risks go up. As we age risks of surgery go up. There are many reasons why the risk of surgery in a 90 yr old is higher. Tissues are weaker; immune system is less effective; healing is slower; getting up out if bed is harder as well. Probably one of the biggest issue is complications. 90 y/o's don't recover from complications as well as a sixty or seventy y/o would. Surgery is possible risks are just higher.
Concern. When the person is 90 years old there are usually co-existing diseases, which can contribute to prolonged recovery.Cardiac, pulmonary systems have lost some capacity and metabolism is much slower. Multitude of factors made your surgeon concerned.
Ageing. Ninety is a grand age and the person who achieves that age has fantastic genetics. But as we age the complication rates for both surgery and anesthesia increase including longer recovery time. If I am lucky enough to reach ninety i would avoid elective surgery and only life threatening conditions should be considered.