Middle adulthood is a. more about aging mentally than physically. b. defined by feelings of chaos and crisis. c. marked by narrowing life options. d. is the time when career options typically widen.

Middle adulthood is  A contemporary phenomenon, basically nonexistent before the twentieth century. 
What considered as 'acceptable' for middle adulthood vary from one culture to another. Before the twentieth century, someone already considerd as 'adults' after they're about 15 years old, so this term did not exist back at that time.