The study was conducted in 2006 by researchers from the world-renowned Department of Growth and Reproduction at University Hospital in Copenhagen, Denmark. The researchers found that among 1,000 girls, the average age of breast development was nine years and 10 months, a full year earlier than when a similar study was conducted in 1991.
"We were very surprised that there had been such a change in a period of just 15 years," researcher Anders Juul said.
Although the study was conducted in Denmark, experts believe that it applies to other parts of the First World, including Europe and the United States. This earlier age of maturation is even more striking when compared with the 19th century, when girls reached puberty at an average age of 15, and boys reached it at 17. Since then, the age of puberty has moved back steadily, until age 14 for boys and age 12 for girls were formally declared "normal" in the 1960s. These numbers were based on the average age of first period for girls and of voices breaking for boys.
It's not just scientific studies suggesting these figures are now obsolete; anecdotal reports of boys dropping out of choir schools when their voices break at age 12 or 13 are now widespread. According to Richard Stanhope, an expert in childhood hormonal disorders, specialists are now convinced that early puberty is a real phenomenon.
Early puberty can be hard on children who are mature physically but still young emotionally, experts warn.
"All the things we experience as teenagers are difficult enough to cope with, but when it happens at 10 or 11 it is much worse," Stanhope said. "These children are also at a much higher risk of being sexually abused, because it is hard for some adults to understand and behave appropriately towards them."
A girl interviewed anonymously by the Times of London said her early development subjected her to teasing at school.
"I had to wear a bra at nine," she said. "I used to pretend to be ill to get out of changing for PE. The worst part was men coming on to me as though I was an adult when actually I was 11."
"If girls mature early, they run into teenage problems at an early age and they're more prone to diseases later on," Juul said. "We should be worried about this regardless of what we think the underlying reasons might be. It's a clear sign that something is affecting our children, whether it's junk food, environmental chemicals or lack of physical activity."
"We don't know if this is the result of better nutrition or environmental factors, but it does create social problems for girls who are already living in a sexualized society," agreed Richard Sharpe, head of the human reproductive sciences unit for the United Kingdom's Medical Research Council.
The cause of early puberty remains contested, but the condition has been convincingly linked both to increased obesity rates and to exposure to endocrine-disrupting toxins such as bisphenol A, the chemical found in plastic water bottles and lining canned foods and beverages.
Supporting both of these hypotheses, a recent study in the journal Public Health Nutrition found that while only 35 percent of girls who ate meat four times a week or fewer had reached puberty by age 12.5, 49 percent of those who ate meat 12 times a week had done so.
Levels of persistent organic pollutants are typically higher in foods high in animal fat, such as meat and dairy.
