Age at onset of puberty varies greatly among individuals and populations and seems to be influenced by both genetic and environmental factors. It follows an approximately normal or gaussian distribution. In girls, the onset of menarche has generally been used as the easiest and most objective indicator for comparative studies. It has been well documented that the average age of menarche has decreased in industrialized northern countries by approximately 2 - 3 months per decade over the last century.

This secular trend toward earlier onset has been related to increased socio-economical level, improved nutrition and better health. Nevertheless, this decrease seems to have been attenuated more recently and to have reached a plateau in Europe and the USA with the average age of menarche between 12.5 and 13 years and Tanner stage 5 reached between 14 and 14.5 years. However, the same authors reported that the onset of breast development or Tanner breast stage 2 (B2) seems to occur earlier than previously thought in the USA, although adequate earlier studies of breast development are not available for objective comparison. Whether environmental factors such as endocrine disrupters with estrogenic effects have any influence on this phenomenon remains to be verified. Mean age of B2 is 9.5 years to 10.9 years and it will take 4 years to reach complete breast development (B5). One precocious event of female puberty is the increase in linear growth and the peak in growth velocity (9 cm/year) reached between stages B3 and B4 at an age of 12 - 14 years.

Menarche will occur at 12.7 years and ovulatory cycles at age 14. Based on epidemiological data, delayed puberty in girls has been defined as the failure to manifest the initial signs of sexual maturation, which begins in girls with breast development, by the age of 13 years (2 SD above the mean for the population) or as lack of menstruation 5 years after the onset of pubertal development with uncompleted development of secondary sex characteristics.

Infantilism is defined by delayed puberty for a bone age of 13 years.

Primary amenorrhea is defined by the absence of menstruation in a 15-year-old girl whether secondary sexual characteristics are absent, partially or completely developed.