Girls are now reaching puberty before age 10 - updated

And that's an average age of puberty - not an outlying limit for precocious individuals:

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.

In the nineteenth century the average age of onset of puberty in females was 15.  By the 1960s it was about 12.  Now it's under 10.

Lots of implications, some of them discussed a different article in The Telegraph:

These girls are towering over boys of their own age because, for girls, the growth spurt and development of breasts come first; periods come later. With boys, it is the other way round: their genitalia and sweaty armpits develop before their height shoots up. The last stage of the maturing process, when they are finally able to signal their manliness, comes when their voices break.

All these markers have been occurring steadily earlier for both boys and girls, but recent changes have been dramatic. In the 18th century, when Bach was directing the Leipzig choir, the average age at which a boy’s voice broke was 18. Choirmasters now have trouble finding trebles over the age of 13 or 14...

Parents, too, should be careful not to treat them as teenagers. “They need to look at their emotional, not their physical, development.

Photo credit PA.

Reposted from 2010 to add new data that suggests the trend is not pathological:

However, our archaeological research suggests that there's nothing to worry about. Children in medieval England entered puberty between ten and 12 years of age – the same as today...

In our study of 994 adolescents from medieval England, who died between 900-1550, we traced the stages of puberty by examining their canine teeth; the shape of their neck and wrist bones; and the fusion of their elbows, wrists, fingers and pelvises. Using these clues, we were able to work out the average age the children started puberty, reached their growth spurt, and reached full maturity. We were also able to work out when girls had their first period. The average age at which children entered puberty was the same as for most boys and girls today: between ten to 12 years. But medieval teenagers took longer to reach the later milestones, including menarche...

Our impression of what is the normal age for a child to reach each puberty milestone has been tainted by the use of data from children growing up in the challenging conditions of the last century, and an over reliance on the age of menarche, rather than the age at which children actually entered puberty, which appears to be unchanged.