The problem with the definition is that if a business has some ancient COBOL code with one guy left who understands it, then your definition would say that is n

The problem with the definition is that if a business has some ancient COBOL code with one guy left who understands it, then your definition would say that is not legacy code. And that is what makes no sense. It becomes legacy when it is no longer running on a tech stack that matches the future strategy of the organization. Legacy is a strategic label, not an engineering label. There is no age where something becomes legacy, it happens when business strategy moves on to something else.