"The age at menarche in a national sample of 4894 Flemish schoolgirls
was surveyed in 1979-1980. The probit estimate of the mean age at
menarche was 13.20 +/- 0.02 years (SD = 1.25 years). ... Status quo
secular data for the 20th century indicate a decline in estimated mean
ages at menarche of Flemish girls from about 14.3 years before World
War II to 13.6 and 13.2 years, respectively, among girls born just
before and during the war. Subsequently, mean ages at menarche of
Flemish girls are fairly stable between 13.0 and 13.2 years. These
secular changes are of the same magnitude as those observed in other
Wellens, R, Malina, RM, Beunen G, Lefevre J. Age at menarche in
Flemish girls: current status and secular change in the 20th century.
Ann Hum Biol. 1990 Mar-Apr;17(2):145-52.