Adolescence seems to be starting earlier and lasting longer in children worldwide. Notably for girls, the age of puberty associated with the development of breasts and other sexual characteristics is creeping steadily downward. Boys are also beginning sexual development anywhere from six months to two years earlier than the medically accepted standard.
Scientists have known for several years that girls and boys are reaching puberty earlier, but they do not know why this is happening, or what the consequences might be. The long term physical and mental health impact of early puberty are also not understood.
The most widely held explanation for this is growing childhood obesity, along with rich diets and lack of physical activity. Other medical theories to explain the phenomenon range from the influence of artificial light to chemicals and hormones in food and the environment and to genetics. Other possible causes include low birthweight, poor diet, lack of activity, widespread vitamin D deficiency, and surprisingly, an absentee father or a non-relative male in the house have also been offered as explanations. Scientists have also been exploring whether so-called 'endocrine disrupters' - a group of chemicals thought to interfere with hormonal function - might be spurring or delaying puberty. Some of these chemicals have retarded sexual development in rat foetuses. Among the most controversial of these chemicals are phthalate esters, which are found in toys, vinyl flooring, detergents, and cosmetics and lotions.
In 1997, a landmark study of 17,000 girls found that the average starting age of puberty had dropped a year from previous studies of Caucasian girls, to 9.7 years. African-American girls reach puberty even earlier, at an average age of about 8. In the past, white girls typically began maturing at age 10, while African-American girls began the process between 8 and 9 years of age. Now, approximately 7 and 27 percent of 7-year-old white and African-American girls, respectively, have either breast or pubic hair development.
German research published in 2012 showed that the onset of puberty for girls had dropped by four years since 1920, and six years over the last century. There are even cases of girls now exhibiting the physical signs of puberty as early as age four!
Overweight and obese girls develop breasts about a year earlier than normal-weight girls. A US study found that adolescent girls who had their first period before their 11th birthday were two times more likely to be overweight than girls who begin menstruating later. There is a theory that too much fatty food in children's diets could be the culprit. As fat accumulates in their tissue, a protein called leptin is secreted. Leptin regulates appetite and appears to trigger hormonal changes that accompany puberty. A 1997 study found that mice injected with leptin matured earlier.