Since the 1960’s the typical progression of sexual development in both boys and girls has been broken into 5 stages called the Tanner stages of development. In boys these stages tend to begin and end at a slightly later age than in girls, so the Tanner Stages in Girls tend to happen at a younger age. This can be a cause of some awkwardness and emotional discomfort for both boys and girls in the 10-18 year age range.
In both boys and girls the Tanner 1 stage is the pre-pubertal state. In this stage for boys there is no coarse or pigmented pubic hair, the penis and testes remain small, and linear growth remains at the basal rate of about 2-2.5 inches (5-6 cm) a year.
Tanner stage 2 in boys tend to continue with their basal rate of increase in height, start having slight increase in testis size and penis length and width, and start having minimal dark coarse hair near the base of the penis. The age range for this anywhere from 9.5-14.5 years.
Tanner stage 3 in boys begin to grow a bit faster, at about 7-8 cm/ year (2.7-3.1 inches) but are still not at their peak growth rate which is later in Tanner stage 4. This is the stage where typically the penis and then the testes significantly, and changes associated with the testis growth and testosterone production are noted. Boys voices begin to change, they sometimes get gynecomastia (breast tissue growty), and start to have some increase in muscle mass. Pubic hair is coarse and pigmented and covers much of the mons pubis. The amount of pubic hair can vary a great deal from one male to another, just as the amount of body hair varies in adult men. This age for this stage can vary from 10-16.5 years old.
Tanner stage 4 in boys is where the fastest growth in height occurs. It typically peaks at 10 cm/ year (about 4 inches) and the penis and testes continue to grow. By the end of Tanner 4 the pubic hair is usually of adult quality and distribution except for not yet being on the inner thigh. Typically in Tanner 4 boys develop coarse axillary hair, start having more acne and their voice change continues. The range of ages for this stage in boys is from about 11-15 years.
Tanner 5 is the adult stage of sexual development. Growth in height is usually complete by about age 17 or earlier in most males. Growth of the penis and testes is usually at adult size by about age 16 ½. Testis size is usually >4.5 cm in greatest length, and young men in Tanner stage 5 usually have facial hair on the sides of the face, any gynecomastia usually resolves, and they develop adult their mature adult male physique. Learning about the tanner stages is an important part of sex education.