With the average life expectancy of a male in the USA being ~75, it makes intuitive sense to call "middle age" 37 +/- some years.
The fact that middle aged is defined as 40-60 is weird in the same way that the "mid-western" United States being almost entirely east of Mississippi river is.
The fact that middle aged is defined as 40-60 is weird in the same way that the "mid-western" United States being almost entirely east of Mississippi river is.