I would imagine that it depends on the quality of life you have already. 80 years old today is not the same 80 it was years ago. My mom is 78 and if she was diagnosed, I would say go for it. My aunt was diagnosed with cancer....not breast cancer...not sure what kind, but she was 82 and her heath had been declining for years. My cousins only kept her comfortable. They never even told her of the diagnosis. She lived another year and a half.
Like Cheryl said, it's definitely a personal decision. Do what ever you have peace about. My mom's good friend was diagnosed in her 70's had chemo, radiation and a mastectomy and she's doing really well.
Blessings to you and what ever decision you make.