We know the age of menarche has decreased in Korea from age 17 to age 12.7 between 1920 and 1986. I think 12.7 is the average age in wealthy nations.
Between 1982 and 2000 Canadian medical school admissions, which are largely grade based, shifted from 50% female to 75% female. The gap in academic performance among median high school students may be even greater. At age 19 boys are far behind girls.
I wonder if the two trends are related.