Girls now reaching puberty before 10 – a year sooner than 20 years ago

The latest generation of girls are reaching puberty before the age of 10, a new study suggests, raising fears they may also begin sexual activity earlier. Scientists have found that the average age that breast development begins is now nine years and 10 months – almost a year earlier than a previous study in 1991.

They have yet to discover the reason behind the phenomenon but believe it could be linked to unhealthy lifestyles or exposure to chemicals in food.

The study was carried out in Denmark in 2006, the latest year for which figures were available, but experts believe the trend applies to Britain.

Data from America also points to the earlier onset of puberty.

Scientists are worried that young girls are ill-equipped to cope with sexual development when they are still at primary school – and that exposure to hormones earlier could increase their risk from breast cancer.

“We were very surprised that there had been such a change in a period of just 15 years,” Anders Juul, head of the Department of Growth and Reproduction at the University hospital in Copenhagen, told the Sunday Times.

“If girls mature early, they run into teenage problems at an early age and they’re more prone to diseases later on.

"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.”

Hitting puberty early can mean longer exposure to oestrogen, which is a factor in breast cancer. There is also a greater risk of heart disease.

A number of artificially produced chemicals have been blamed for interfering with sexual development, notably bisphenol A, a plastic found in the lining of tin cans and babies’ feeding bottles.

Mr Juul’s research team is now testing blood and urine samples from girls in the study to see if a direct link can be drawn between early sexual maturation and bisphenol A.

Another factor in puberty could be diet. Children are eating more than previous generations and growing bigger — and in many cases becoming obese.

There has been a steady lowering in the onset of puberty. In the 19th century, it was at about 15 for girls and 17 for boys.

The international standard for normal puberty in white girls was set in the 1960s at 12 for the age when periods begin and at about 14 for boys when their voices break and their growth surges.

A more recent consensus in Britain has proved less conclusive.

“Although we don’t have clear data here, there is evidence the same thing [as in Denmark] is happening for reasons that we don’t understand,” said Richard Sharpe, head of the Medical Research Council’s human reproductive sciences unit in Edinburgh.

“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 sexualised society.”