Tanner scale

The Tanner scale (also known as the Tanner stages or Sexual Maturity Rating (SMR)) is a scale of physical development in children, adolescents and adults. The scale defines physical measurements of development based on external primary and secondary sex characteristics, such as the size of the breasts, genitals, testicular volume and development of pubic hair. This scale was first identified in 1969 by James Tanner, a British pediatrician, after a two-decade-long study following the physical changes in girls undergoing puberty.
Due to natural variation, individuals pass through the Tanner stages at different rates, depending in particular on the timing of puberty. In HIV treatment, the Tanner scale is used to determine which regimen to follow for pediatric or adolescent patients on antiretroviral therapy (adult, adolescent, or pediatric guidelines). The Tanner scale has also been used in forensics to determine aging, but its usage has decreased due to lack of reliability.
Definitions of stages.
Adapted from "Adolescent Health Care: a practical guide" by Lawrence Neinstein, M.D.
Pubic hair (both male and female).
Sometime during Tanner 5, females stop growing and reach their adult height, usually this happens in their mid teens at 15 or 16 years for females. Males also stop growing and reach their adult height sometime during Tanner 5, usually this happens in their late teens at 18-19 years for males.
Criticism.
The scale has been criticized by the pornography industry for its potential to lead to false child pornography convictions, such as in the case when United States federal authorities used it to assert that pornographic actress Lupe Fuentes was underage. Fuentes personally appeared at the trial and provided documentation that showed that the DVDs in question were legally produced. 
Tanner, the author of the classification system, has argued that age classification using the stages of the scale misrepresents the intended use. Tanner stages do not match with chronological age, but rather maturity stages and thus are not diagnostic for age estimation.