Children are going through puberty at an increasingly early age, and the changes to their bodies are also affecting their mental health, new research says.

Biological changes are happening earlier in children around the world - in 1860, the average age for European girls to develop breasts was 16.6 years. In 2010, the average age was 9.9 years, according to a United States study.

Other recent studies out of the US have found boys as young as 6 and girls as young as 8 showing the first signs of puberty.

An Australian study published yesterday has found that early puberty is associated with poorer mental health. It could also be why more people were suffering mental health problems later in life, Professor George Patton, of the Murdoch Children's Research Institute in Melbourne, said.

The study also found that boys who started puberty before their ninth birthdays suffered from behavioural difficulties, but girls did not.

Wellington Hospital paediatrician Esko Wiltshire said anecdotal evidence showed children in New Zealand were also experiencing puberty earlier than in previous generations.

"Early puberty by definition is girls starting before 8 and boys starting before 9. Anecdotally, we are seeing more children presenting with these problems."

For some, it could be hormone-related and treatable, but for others nothing could be done to stop their bodies maturing, which meant they looked older than they were.

The challenges around an 8-year-old looking like a 12-year-old were significant because their cognitive development was still at the same level as their peers, Dr Wiltshire said.

The Melbourne study found that children who showed definite signs of puberty at the age of 8 or 9 had poorer emotional and social adjustment.

Lead researcher Fiona Mensah said there was a higher risk of behavioural and emotional problems during puberty, and children who matured earlier had more of these difficulties in adolescence.

The children who began puberty earlier also had poorer mental health when they were aged 4 or 5. Dr Mensah said this suggested the link between early puberty and poorer mental health was due to "developmental processes" that started early in life.

The earlier that puberty kicked in, the longer the "window of vulnerability" was open, putting children at greater risk of depression, self-harm and anxiety, Prof Patton said.

The research, which followed nearly 3500 children in a longitudinal study, supported the suggestion that genetic and environmental factors in early life could affect the timing of puberty, he said.

Better nutrition in childhood is widely believed to play a role in the early onset of puberty, but poor nutrition during pregnancy may also trigger it.

The paper, Early Puberty and Childhood Social and Behavioral Adjustment, is published in the Journal of Adolescent Health today.

- The Dominion Post