How old is middle age?

This is a vague age group, which can only be roughly determined according to everyone's life span, but can your fortune-telling be so accurate? For example, the life span of 80 years can be pushed to 1-20, that is to say, you are a boy who divines, 2 1-40, a middle-aged man who divines, 4 1-60, a middle-aged man who divines, and 6 1-80.