In the late 1980s, Marcia Herman-Giddens was working in a paediatric clinic at Duke University Medical Centre in North Carolina when she noticed a puzzling phenomenon. More and more girls aged eight or nine who visited the clinic had started to sprout pubic hair and breasts. At the time, medical orthodoxy held that the average age of puberty for girls in the west was over 11. The numbers of under-10s that Herman-Giddens was seeing did not fit with this scenario. She began collecting data that eventually produced a study with the American Academy of Pediatrics that studied 17,000 girls and found that the average age of breast-budding among white girls was 9.9 years while for black girls it was 8.8.

The discovery was hugely controversial. Many doctors refused to accept the fact that more and more girls had begun to mature sexually before they had reached the age of 10. "The Lolita syndrome [the prurient fascination with the sexuality of young girls] created a lot of emotional interest," recalls Herman-Giddens, now at the University of North Carolina. "As a feminist, I wish it didn't."

Today most doctors accept that the age of onset of puberty is dropping steadily. Many studies have showed this to be the case for girls, and new research carried out by Herman-Giddens, and published by the American Academy of Pediatrics, has found the same for boys. The age of onset of biological adulthood continues to plunge. Consider the statistics provided by German researchers. They found that in 1860, the average age of the onset of puberty in girls was 16.6 years. In 1920, it was 14.6; in 1950, 13.1; 1980, 12.5; and in 2010, it had dropped to 10.5. Similar sets of figures have been reported for boys, albeit with a delay of around a year.

What factors lie behind this trend? Why are our children reaching biological adulthood at earlier and earlier ages? And what are the medical implications of this? Answers to these questions are still debated, although most scientists and health experts believe that the initial decline in the age in puberty was linked to general improvements in health in the west that began in the late 19th century. The trouble is that this drop, which was expected to stop, has simply continued at the same rate: a decline in four to five months in age of onset for each passing decade.

This relentless slide has begun to worry doctors who have proposed a host of causes to explain it. Increasing obesity is often quoted. In the young it is thought to increase blood levels of oestrogens that promote breast development and early studies seemed to confirm this by linking puberty to higher body mass index. However, a Danish study released last year in the journal Paediatrics found puberty occurring earlier in children regardless of body mass index at age seven.

Other factors that may be involved include a diet that is increasingly high in sugar and fat and declining physical activity. The cause could also be environmental, say other researchers – in particular, exposure to endocrine disrupters, chemicals in the environment that act on hormones.

Widespread industrial and pharmaceutical pollutants have already been shown to harm the normal sexual development of fish and animals. By extension, they may also contribute to earlier or disrupted puberty in children, these scientists contend.

As to the likelihood that precocious puberty poses perils for young people, this is still debated – though many parents worry that early sexual development puts strain on children who are being robbed of years of innocent childhood.