Our doctor is suggesting amputation for my 69-year-old grandmother's leg deep wound. Isn't she too old for an operation?

Not necessarily. In medicine we make decisions based on risk assessment. If it is more risky for your grandmother to keep the leg wound than to have her leg amputated, then an amputation is the right decision. Age alone is not a contraindication to any procedure. As long as she does not have any other medical co-morbidities, surgical risks can be managed.
No. People at that age can have surgery successfully. The main issue is not the age but the overall health of the person at the time of surgery. Amputation surgery can be life saving and with a good prosthetic many people are more mobile than they were with a non-healing wound.
Amputatijn. No, her age is not an absolute barrier. Hav e her primary care physician evaluate her medical problems and communicate with her surgeon.