Millennials are stuck in adolescence until the age of 24, experts reveal

Adolescence – the phase of life stretching between childhood and adulthood – has long proven difficult to define with precision.

Traditionally, it has been thought to last between the ages of 10 and 19, however, in a fast-changing world, perhaps a more appropriate range would be 10 to 24 years old, according to an opinion piece published in the journal Lancet Child & Adolescent Health.

"Adolescence encompasses elements of biological growth and major social role transitions, both of which have changed in the past century," the authors write in the study.

"Earlier puberty has accelerated the onset of adolescence in nearly all populations, while understanding of continued growth has lifted its endpoint age well into the 20s."

They argue that with young people staying in education longer and delaying other major milestones, such as marriage and parenthood, the popular perception of when adulthood begins has shifted.

It is important, they say, to change the definition to ensure that the law can keep pace with societal changes.

"Arguably, the transition period from childhood to adulthood now occupies a greater portion of the life course than ever before at a time when unprecedented social forces, including marketing and digital media, are affecting health and well-being across these years," the researchers explain.

"An expanded and more inclusive definition of adolescence is essential for developmentally appropriate framing of laws, social policies, and service systems."

In the UK, the law defines adulthood as beginning at 18, however, adult responsibilities and true economic independence tend to occur later for most people.

This is highlighted perhaps most effectively by the fact that the average age for men and women to get married is now around eight years later than it was in 1973 (32.5 years old for a man and 30.6 for a woman), according to the Office for National Statistics.

The authors argue that redefining adolescence will help to create more effective policies, such as raising the age at which youth support services can be accessed to 25.

There are also biological factors to consider, including the fact that the brain and body both continue to develop after 18.

Furthermore, puberty - which is initiated by the release of hormonal signals from the brain - is now happening earlier in many more affluent countries as a result of improved health and nutrition. In the UK, for example, a girl will now begin her period four years earlier than 150 years ago.