By Michelle Roberts
BBC News health reporter
Whether it is sprouting hair, budding breasts or a breaking voice, the signs that herald puberty can be distressing and difficult to cope with.
Puberty age is going down
In the western world there is much evidence children are reaching puberty at younger and younger ages - some girls at the age of seven.
The reasons for this trend are unknown - and some dispute it is occuring at all.
But several theories have been suggested.
And Swedish scientists at the Karolinska Institute aim to find out by tackling the puzzle from different angles.
It is accepted that the normal age for a girl to begin to develop the first signs of puberty is 10 and above. Boys develop slightly later, generally at eleven-and-a-half.
However, the age appears to have been decreasing in developed countries.
In 1990, the first signs of puberty were around the age of eight for girls - the whole process taking two years to complete.
Now, according to researchers, some enter puberty as young as seven.
Boys, too, say some experts, are entering puberty at an earlier stage, albeit still slightly later than girls.
But it is unclear whether this is a simply a shift of the norm, or if more children are experiencing a phenomenon called precocious puberty - when they develop the first signs of puberty abnormally early.
Controversial theories have been put forward, including watching too much television could distort the hormonal balance of adolescents and push many of them into early puberty.
Psychologists have said young girls who have close relationships with their fathers might enter puberty later than girls with distant or non-existent links.
Now 12 European teams are carrying out research as part of a three-year project to get to the root of the problem, looking at the most likely culprits.
Professor Olle Söder from the Karolinska Institute is leading one study which will look at whether rising obesity rates are to blame.
His team will study whether animals that are overfed produce more of the male and female sexual hormones that trigger puberty.
"We believe that this has a nutritional background and that the obesity explosion we have seen in the US, and which is coming to Europe, is important," he said.
Colleagues in Germany will gather data on around 50,000 children to look at whether those who are plumper reach puberty earlier.
A London group will look at strains of mice renowned for early or late onset of puberty and see whether they can modify this with diet.
Researchers have shown that overfed and rapidly growing newborn babies go on to reach puberty earlier than other babies.
Also, adoption studies show undernourished children who have catch-up growth after being placed with more affluent families have earlier onset of puberty than siblings who remain in their home place.
Pesticides and pollution
"Another thing that might be important is environmental factors that mimic hormones, such as pesticides," said Professor Söder.
A team of Belgian researchers pointed the finger at a chemical derivative of the controversial pesticide DDT.
Jean-Pierre Bourguignon and colleagues from the University of Liege found children who had emigrated from countries such as India and Colombia were 80 times more likely to start puberty unusually young.
Three-quarters of these immigrant children with "precocious" puberty had high levels of a chemical derivative of DDT in their blood.
However, there is no firm evidence. Some of the European researchers will probe this further.
Equally, he said it might be down to genetics.
A team of researchers in the UK and the US recently pinpointed a gene that they believe controls puberty through the regulation of a protein called GPR54.
The US scientists, from Massachusetts General Hospital, found that the gene that codes for GPR54 was mutated in all members of a Saudi Arabian family who failed to reach puberty.
At the same time, scientists at the UK biotechnology company Paradigm Therapeutics contacted the US doctors to tell them they had bred mice that had failed to reach puberty.
They had "knocked out" the gene for GPR54 in these mice.
Regardless of whether it is down to one factor or many, it is not clear whether children entering puberty earlier is a problem, said Professor Söder.
Some say girls who reach puberty earlier are more likely to drop out of school and have lower incomes.
Data shows that they are also more likely to become mums earlier.
With more and more women putting off having a baby until later life, this might be a good thing and help reverse trends of top heavy ageing population, said Professor Söder.
But equally, it might mean more women reach the menopause earlier too and miss their chance to become a mum, he said.
Either way, "it will have a societal impact," he warned.