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Twenty-four: is this the age we finally grow up?

They should be the healthiest people on the planet, the "almost grown-ups" still in the bloom of youth and full of dreams for the future.

But today's adolescents are instead a troubled generation, marooned in a no man's land between childhood and adulthood, prey to forces beyond their control.

Far from being the healthiest time of life, adolescence is instead a period of maximum risk and maximum vulnerability, according to scientists, as still-growing bodies and undeveloped minds hurl themselves into experimentation with drink, drugs and sex.

They are targeted by the mass marketing of unhealthy products and lifestyles -- tobacco, alcohol, junk food -- which doctors compare to an infectious disease epidemic.

And evidence now shows that teenagers are among those exposed to the greatest threats.

In a series of papers in medical journal The Lancet, scientists describe how new research has changed our understanding of adolescence, which was thought to start with the physical changes to the body around puberty and to be completed when growth stopped in the late teens.

Now researchers believe the brain goes on maturing and is not fully developed until at least the age of 24 -- a milestone recently hit by the likes of superstar singer Rihanna.

While puberty catapults adolescents into a period of risky behaviour powered by their raging hormones, their brains are ill-equipped to exert control and are vulnerable to the effects of drinking and drug taking.

Scientists say the adolescent brain is handicapped in the rational assessment of risk and prone to "hot cognitions" -- decisions influenced by exciting or stressful conditions that adults are better able to resist.

The modern teenager is taking longer than ever to grow up as social changes have added years to eduction and delayed marriage and "settling down".

Despite its legal significance, 18 is no longer regarded as the start of adulthood.

Some 50 years ago, people married and started families at this age. Today, these milestones are delayed, over three times more young people are in further education and most spend years in work before marrying and settling down.

The single biggest cause of deaths in adolescence is injury as a result of accident, often triggered by unnecessary or excessive risk-taking.

Better scientific understanding of the maturation of the brain has underpinned policy decisions such as the limit imposed in the United States and Canada on the number of adolescent passengers permitted in a car with a young driver.

Yet despite its significance for life-long health, adolescence has been neglected in scientific and medical research.

The health of under-fives has improved dramatically in the past 50 years, with death rates down by 80pc in many countries. But adolescent health has only marginally improved.

US researchers warned last year that heavy drinking during the teenage years, when the brain is still growing, can affect the brain, particularly the development of spatial memory -- the ability to orientate oneself on a map and recall how to get from place to place.

Professor George Patton, from the University of Melbourne, Australia, and colleagues said new estimates of teenage drinking expected in the next few months in many countries would "provide an opportunity to assess whether policies to reduce harmful drinking among teenagers have had any impact over the past five years".

They add: "For the largest generation in the world's history, the available global profile of youth health is worrying."

Yet for the 1.8bn adolescents in the world today, comprising more than a quarter of the world's population, families remain the key factor in healthy development.

Adolescents who feel connected to their family smoke fewer cigarettes, drink less alcohol, use less drugs, start sex later and are less likely to be involved in violence.

Parents who know about and are involved in their children's lives are less likely to have problem adolescents.

Research suggests infancy and early adolescence represent two clusters of risk.

The abused or neglected child, struggles to learn in primary school, is rejected by its peers and then progresses to drinking, smoking, drug taking, early sexual experience, violence and pregnancy, leading to the premature end of schooling.

More than half of those who develop disorders such as alcohol misuse and anti-social personality disorder are diagnosed by the age of 25, highlighting the vulnerability of the adolescent years.

Yet the damage can be limited by effective policies such as providing free contraceptives, increasing taxes on alcohol and imposing restrictions on driving.

The Lancet says four steps are needed to improve the lives of adolescents: international comparisons of their behaviour, goals for their healthy development, preventive measures to reduce obesity, alcohol and drug abuse, and monitoring of their progress.

It adds: "Young people are our future assets. They provide energy, innovation, productivity and progress... Adolescent health [should be] an equal concern alongside existing health priorities in the world."