page_title: Puberty
section_title: Age of onset
hierarchical_section_title: Puberty / Variations / Age of onset
caption_attribution_description: Français&#160;: Développements pubertaire comparés de deux filles, dont l'une avec une puberté tardive English: Comparison of the puberty of two girk, among which one with delayed puberty
context_page_description: Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Before puberty, the external sex organs, known as primary sexual characteristics, are sex characteristics that distinguish boys and girls. Puberty leads to sexual dimorphism through the development of the secondary sex characteristics, which further distinguish the sexes.
  On average, girls begin puberty at ages 10–11 and complete puberty at ages 15–17; boys generally begin puberty at ages 11–12 and complete puberty at ages 16–17. The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13. For males, first ejaculation occurs on average at age 13.
context_section_description: The definition of the onset of puberty may depend on perspective (e.g., hormonal versus physical) and purpose (establishing population normal standards, clinical care of early or late pubescent individuals, etc.). A common definition for the onset of puberty is physical changes to a person's body. These physical changes are the first visible signs of neural, hormonal, and gonadal function changes.
  The age at which puberty begins varies between individuals; usually, puberty begins between 10 and 13 years of age. The age at which puberty begins is affected by both genetic factors and by environmental factors such as nutritional state and social circumstances. An example of social circumstances is the Vandenbergh effect; a juvenile female who has significant interaction with adult males will enter puberty earlier than juvenile females who are not socially overexposed to adult males.
  The average age at which puberty begins may be affected by race as well. For example, the average age of menarche in various populations surveyed has ranged from 12 to 18 years. The earliest average onset of puberty is for African-American girls and the latest average onset for high altitude subsistence populations in Asia. However, much of the higher age averages reflect nutritional limitations more than genetic differences and can change within a few generations with a substantial change in diet. The median age of menarche for a population may be an index of the proportion of undernourished girls in the population, and the width of the spread may reflect unevenness of wealth and food distribution in a population.
  Researchers have identified an earlier age of the onset of puberty.  However, they have based their conclusions on a comparison of data from 1999 with data from 1969.  In the earlier example, the sample population was based on a small sample of white girls (200, from Britain).  The later study identified as puberty as occurring in 48% of African-American girls by age nine, and 12% of white girls by that age.
  One possible cause of a delay in the onset of puberty past the age 14 in girls and 15 in boys is Kallmann syndrome, a form of hypogonadotropic hypogonadism (HH). Kallmann syndrome is also associated with a lack of sense of smell (anosmia).  Kallmann syndrome and other forms of HH affect both men and women. It is caused by a failure in HPG axis at puberty which results in low or zero gonadotropin (LH and FSH) levels with the subsequent result of a failure to commence or complete puberty, secondary hypogonadism and infertility.
  Comparison of two individual with vast difference in the age of onset of puberty :
caption: Two girls from 8.0 to 14.5 years old
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