Scientists say they have found the genetic switch that turns on puberty.
Some children can hit puberty earlier than others
It all starts with a molecule called kisspeptin, which wakes up the reproductive hormones from their childhood hibernation.
The University of Pittsburgh team believe kisspeptin switches on a recently discovered puberty gene, called GPR54.
The findings could help treat puberty disorders, they told Proceedings of the National Academy of Sciences.
The team found that when they gave young animals kisspeptin it triggered the production of reproductive hormones.
It might be possible to do the same in humans to treat children who fail to go through puberty in the normal way, they believe.
About one in 10,000 children fail to go through puberty. Many others go through puberty prematurely - some at just two years old.
Lead researcher Dr Tony Plant said: "We now have very good evidence that the GPR54 gene and its switch, the kisspeptin protein molecule produced by kiSS-1 [gene], are key to the initiation of puberty."
But he said it was likely that other signalling systems were also involved.
Professor Ilpo Huhtaniemi, an expert in reproductive biology at Imperial College London, said: "This is a very important field of research.
"Puberty is advancing in Western societies all the time and we do not know why.
"These new receptors and their peptides might play a role. Diet is another factor."