Puberty in boys

A good source of information below:

Boys and girls begin and end puberty at all different ages. What is considered 'normal' or typical varies a great deal. To get a better idea of how much variation is considered normal, it is important to understand how puberty is assessed.


Puberty, the natural progression from a child-like physique to an adult body, begins with hormonal signaling between the gonads (the testes in the case of boys) and a part of the brain called the hypothalamus and the pituitary gland. In boys, puberty usually begins between 9 and 14 years of age in the United States. (This is an important point to make as ethnicity apparently influences the average age at which puberty begins both for boys and girls.) If there is evidence of testicular enlargement before the 9th birthday or no evidence of pubertal changes by the 14th birthday, then those are reasons for concern and must be evaluated.
The first physical change that is noted in a boy is the enlargement of the testes and the lengthening of the penis. In the second stage of pubertal development, the skin over the scrotum begins to thin and redden and there is the beginning of a sparse amount of pubic hair at the base of the penis. Because most parents are no longer seeing their children fully undressed in early adolescence, these very early changes in boys may not be noticeable to the parent.


The third stage of puberty begins when there is increased amount of pubic hair that is noticeably coarser and darker than before. It is at this point that the peak height velocity or 'growth spurt' usually begins. This is the most obvious physical change and is typically about two years after the onset of the very first signs of puberty. The 'growth spurt' lasts about two to three years. A boy achieves about 25% of his final adult height during the growth spurt, and he will gain an average of 3.5 inches a year during this time. This compares to 2.3 inches per year in the prepubertal male. Another common occurrence in the third stage of puberty is the development of breast tissue. This is called gynecomastia, and it can be on one or both sides. For most boys this is a passing change that will spontaneously improve if the breast tissue is less than 1.5 inches across.


After the onset of the third stage of puberty, boys also gain in muscle mass, the voice deepens, acne frequently becomes bothersome. The fourth and fifth stages of pubertal development are marked by increased in pubic, axillary, and body hair as well as further enlargement of the penis and testes. For boys there is also a 'strength' spurt related to the increased muscle mass. Final height is usually achieved during the final stages of puberty. A look at the growth charts for boys ages two to eighteen can give you some sense of where your son fits in with his peers with regard to height and weight.

Source:
http://www.keepkidshealthy.com/development/puberty_boys.html

Puberty in girls

This is  a good article on puberty development in girls. Important points are highlighted in bold.

Normal puberty in girls is a complicated topic. To understand what is normal, we need to start with some basic information. Before any visible changes take place, hormonal changes are occurring for a couple of years. These hormonal changes start in the adrenal glands, two small organs that rest on top of either kidney. These hormones send signals to an area of the brain known as the hypothalamus and the pituitary gland. The hypothalamus and the pituitary, in turn, send hormonal signals to the gonads, which in the case of girls, are the ovaries. This system of signaling takes a couple of years to become fully established, but once it does, then the physical, visible changes that we call puberty begin to take place.


Puberty follows a fairly consistent sequence in girls. Doctors stage the physical changes on a scale of 1 to 5, called Tanner staging. Children who do not show any physical changes of puberty are at Tanner stage 1. Adults who have completed puberty are at Tanner stage 5. For girls, Tanner staging is done to assess both breast and pubic hair changes. Sometimes the changes in these two areas are not at the same Tanner stage at the same time, and that is okay.


The first visible changes are usually the development of breast buds (known as thelarche). This usually occurs between ages 7 to 13 and marks the transition to Tanner stage 2. Again ethnicity plays a factor in the age at which puberty first appears. Up to 15% of African-American girls with have breast buds by age seven years. Percent body fat is another factor associated with the earlier onset of pubertal changes. However, many overweight girls only appear to have breast development because of increased body fat deposited on the chest. This “pseudo” breast development can usually differentiated from real breast development by a doctor during the physical exam. Breast budding may begin on one side before the other and is often accompanied by tenderness. The tenderness generally goes away in a few months.


Following breast development, most girls then have the development of coarse, dark pubic hair (adrenarche) generally limited to the labia majora. This is also Tanner stage 2. Some girls (about 15%) will have the development of pubic hair before breast development. The amount of pubic hair increases to an almost adult amount prior to the onset of the menses (Tanner stage 3).


The onset of the menstrual cycles (menarche) begins about 2 to 2 1/2 years after the onset of breast development. Thus, the menstrual cycle can occur anywhere from age 9 to age 15 and be considered 'normal.' Most girls do not have regular, predictable menstrual cycles for another year or two.
Most girls have their growth spurt in the year preceding the onset of the menstrual cycles and have reached closed to their final adult height, unlike boys who have their 'growth spurt' later in puberty. Most girls stop having bone growth within 18 to 24 months after the onset of menses.

Source: http://www.keepkidshealthy.com/development/puberty_girls.html