Boys are hitting puberty six months to two years earlier than they used to, according to US research.
While it's generally been accepted that boys hit puberty around 11 and a half, this study found most boys actually start maturing at 10.
In the study, more than 4,100 boys from 41 US states were monitored by pediatricians.
George Patton, professor of adolescent health research from the University of Melbourne, said the study shows boys hit puberty earlier than we first thought –– but says that doesn't mean the age of puberty onset is lowering.
"The original study that the authors were comparing it with was done in the UK back in the 1970s with a small number of kids in institutional care," Professor Patton said.
"The kids might have had a whole lot of things in their development that might have affected the onset of puberty. This was the only study we had for many years so people kept going back to it."
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Professor Patton said the new study was much more comprehensive.
"These people in the US are saying, compared to the original study that was done with a different group, in a different country and a different methodology, we think the age of onset of puberty is lower –– but that doesn't mean to say it is falling," he said.
"This is a very good study of puberty and it looks as if the age of onset of puberty in boys is an earlier process than what we thought it was."
However Professor Patton said there has definitely been a shift in the past 200 years.
"Compared to generations 200 years ago, this is definitely much earlier," he said.
"The mean age of the onset of periods for girls was 17 and a half, compared to 12 and a half in the mid 20th century –– so we lost about five years. But since the mid-20th Century, it's been much harder in high income countries to see a major change in the onset of puberty."
Professor Patton said a range of factors could have contributed to the age change in the past 200 years –– but the most likely influence is nutrition.
"It's better nutrition for the individual, plus better nutrition for their mothers and their grandmothers before them, so it's probably an accumulated process," he said.
The study was published in the journal Pediatrics.