What does it mean if the physician tells me I'm too old to have a foot amputation?

Not sure ... I'm not sure how at 55 you would be too old to have an amputation. Perhaps they mean your general health or your circulation is poor. Discuss it with your physician and have them clarify why they feel that you are not a surgical candidate.
Non By-passalbe. At 54 youre not old for foot amputation. I would get a 2nd opinion. If you are non bypassalbe, meaning that your arteries can not be by passed to increase blood flow to your foot, then may be you can not have a foot amputation and higher amputation will be better.
Too Sick? I'm assuming you may have too many other medical problems for a foot amputation. Sometimes anesthesia itself is risky, especially in people who have active heart disease or lung disease. Age by itself does not exclude anyone from a procedure. The question we always ask in medicine is, "is the risk of not doing anything greater than surgery?" if yes, then we proceed with surgery.