PPT Slide
Frequency distribution of menarcheal age and age at conception of a sample of African-American adolescents (n=574)
Rees JM, 2001 (unpublished)
Notes:
Note how the population is arrayed, with a longer tail on early end.
Also see the relationship of early conception to the age of menarche.
Early conception is one of the concerns related to early sexual maturation.