The average age of a girl at menarche (onset of first menstrual period) is decreasing worldwide; women now reach sexual maturity at a significantly younger age than their grandmothers. Determining the factors related to this trend is critical, as it is occurring globally and may be markedly different between industrialized and non-industrialized populations. In addition, addressing the consequences of an early age at menarche is essential, as there are many biological, social, and psychological problems related to an early age at menarche. Trends in age at menarche, sexual debut, and oral contraceptive use are compared between and among Swedish and Swedish Symi women who were born between 1943 and 1990 and who live in Norrbotten or Vysterbotten counties. The comparison of these populations is advantageous because many Swedish Symi families have, in recent generations, made the transition from nomadic reindeer herding to living and working in industrialized areas. Possible explanations regarding differences and similarities between the two groups in terms of age at menarche, sexual debut, and oral contraceptive use, as well as the role of sexual education and access to health care, are discussed.