No, it does not assume that at all
“Middle age” has always meant the years from when you are at about half of life expectancy until you go to the next stage of life of newr-retirement or retirement
Aka 40s-60s
I had boldly assumed that it meant middle of life expectancy, but it does not (although exact range can be debated): https://en.m.wikipedia.org/wiki/Middle_age#:~:text=Middle age%2C or middle adulthood,young adulthood and this stage.