Australian teenagers face more risks from binge drinking, unsafe sex and junk food because they are delaying adult behaviour such as marriage.
Instead, they extend their adolescence well into their 20s, an international study has found.
Research papers in The Lancet today, some written by Australian experts, argue that instead of puberty being a brief period of turmoil between the ages of 12 and 17, adolescence in Western countries was spanning from ages 10 to 24.
This was about 10 years longer than previous generations and meant the legal age of 18 no longer signified adulthood.
Children were "growing up earlier" by having sex and drinking younger but were putting off settling down and taking on adult roles.
This put them at greater risk from adolescent "bad behaviour".
Researchers said that 50 years ago it was common for people to settle down and have a family in their early 20s.
Today, these milestones were delayed as Facebook, Twitter and urbanisation changed traditional family and community influences.
Professor Susan Sawyer, of Melbourne's Murdoch Children's Research Institute, said this was having a profound effect in countries such as Australia, which had a "youth deficit", making up only 18 per cent of the population.
Efforts were made to prevent health problems in early childhood and puberty but those aged 18 to early 20s were in a no-man's-land.
"Young people are a resource for national wealth but we're seeing their adolescence going on for a much longer period and starting much earlier," she said.
"This is having a profound effect in terms of their health risks
"Fifty years ago, the timing of first sexual intercourse and marriage was about the same in Australia - the age of 20 or 21 - but now sex begins at the age of 16 and girls are getting married at 29."
Professor Sawyer said social media put young people in of danger from cyberbullying and "sexting".
The nature of social contagion, when ideas or images could spread easily, had implications for copy-cat suicides, school shootings and disturbed body image.
Social media also contributed to a new level of sleep deprivation in youths that could affect many aspects of their lives profoundly.