Children should be given advice on adult issues much sooner because of earlier puberty, a report says.
Researchers say children now reach puberty earlier than ever
It says social problems such as alcohol abuse, unprotected sex and self harm may result from children receiving information on these topics too late.
The study, from Liverpool John Moores University Centre for Public Health, says the age at which puberty begins has come down for the past 150 years.
It is published in the Journal of Epidemiology and Community Health.
The report says there have been no attempts to develop young people faster, leaving "an increasing gap between physical puberty - changes to their bodies - which tends to happen at around 12, and social puberty - when they are able to make decisions for themselves".
This, it argues, can result in unprotected sex, substance abuse, self harm, violence and bullying.
Dr Mark Bellis, one of the report's authors, said: "Children who are now developing at an earlier stage need a different approach.
"Information often regarded as adult on sex, relationships and dealing with issues of conflict is currently given to them when it is too late.
"The gap between when children are developing and coming into adulthood and when adult information is dispensed is the longest it's ever been.
"So giving it over at a later stage can do more harm than good because children need to get to grips with the changes in their life sooner."
Dr Bellis said, because of this delay in receiving adult advice, children can develop problems with sexual health, substance abuse and violence because they are not well-enough equipped to deal with the difficulties they face in life.
"What we don't know, and what we should be looking at, is do we have children developing early in poorer areas, which often have a high number of single parent families without a father.
"We cannot say that puberty is definitely happening earlier in poor areas.
"But what we are saying is that there are all three factors in these poorer areas - social problems, single-parent families and a high birth rate."
He said better nutrition and the reduction in child health diseases were "public health reasons" for the earlier development of puberty.
But he added that another factor was the absence of a father in the parental home.
"Recent studies in America show a strong relationship between a tendency for puberty to happen earlier when a father is not present.
"We can't judge children in the 21st century the same way that we did 100 years ago, because they don't look the same and are at a different mental stage than they were back then."
Gary Butler, professor of paediatrics at the University of Reading, said there was a need for sex education to be given earlier, but not because of puberty issues.
"Giving information earlier needs to be done anyway but not as a result of the age children reach puberty.
"Puberty may be starting sooner but there's no difference in the age at which children become fertile.
"We need to tackle the issue of teenage pregnancies but that's separate from the biological issues."