Puberty is the period during which the reproductive organs mature and reproduction is made possible. It usually starts at the age of 10 for girls and 13 for boys, and it completes at around 18.
In male, puberty is characterized by rapid growth and widening of shoulders. The testes and scrotum begins to develop at an accelerated rate which leads to accelerated penis growth on the average at 13-15 years old. Pubic hair appears later on, followed by axillary and facial hair. These are caused by increased GnRH secretion which leads to increased level of gonadotropins. Gonadotropins are hormone which stimulate the seminiferous tubules and testosterone secretion. Testosterone not only induces spermatogenesis, but also triggers off development of accessory reproductive organs, sex characteristics, and sex drive.
The breasts start to grow at a faster rate in girls at the age of around 11 which marks the beginning of puberty. This is usually accompanied by the appearance of pubic hair and the widening of hips. Menarche, the first menstrual period, appears later on, about one year afterwards. This is usually the time when the peak of the total-body growth spurt has just passed. These changes are, analogous to that for boys, caused by the increased GnRH secretion by the brain. This hypophysiotropic hormone stimulates pituitary gonadotropins secretion, which induces follicle development and estrogen secretion. Estrogen is mainly responsible for follicle maturation, and also contributes to the striking development in the accessory sex organs and secondary sex characteristics.
Although the exact age at which puberty occurs varies among individuals, the maturational events of puberty proceed in an orderly sequence. The process is not abrupt but develops over several years. This is evident on the slowing rising gonadotropins and testosterone or estrogen levels in the blood.
Humans remain sexually active after puberty when they are able to reproduce. However, this reproductive function will cease ultimately when menstrual irregularity and decline in sex desire begin to be observed at the age of around 50. This cessation is known as menopause which involves plenty of physical and emotional changes, as a result of hormonal changes.
As a female is aging, the number of ovarian follicles remaining in her body decreases and they become hyporesponsive to gonadotropins. This causes ovarian failure which leads to decreased concentration of estrogen in plasma as estrogen is secreted by ovary. Finally, estrogen concentration becomes too insufficient to maintain the estrogen-dependent tissues. The breasts and genital organs begin to diminish and the vaginal epithelium becomes thin and dry, causing irritation in sexual intercourse. There will also be a drastic decrease in bone mass and strength which may further leads to bone fracture.
In male, the effect of menopause is less significant. At the age of around 40, testosterone level decreases gradually. This results in slow deterioration of testicular activities and failure of the gonads to respond to the pituitary gonadotropins. Although sex drive and capacity diminish with age, men are still fertile at their elderly.