Why surgeon says age (of 90) is big issue for doing surgery.?

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.