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If your 20-year-old son was still struggling to pull himself out of bed this morning, there may be a reason.

Scientists say adolescence now lasts until the age of 24 because the young are postponing the responsibilities of adulthood.

Traditionally adolescence, which follows the onset of puberty when a young person develops into an adult, was thought to end at 19 but researchers from Australia say this definition is too restrictive.

Puberty is also starting earlier thanks to improved nutrition.

Writing in the Lancet, the scientists say many young people are delaying adulthood by continuing in education and putting off marriage.

They want the definition of adolescence to be expanded to the years between ten and 24 to ensure young people receive support from the Government. Professor Susan Sawyer, of the Royal Children’s Hospital in Melbourne, said: ‘The adoption of adult roles generally occurs later.

‘An expanded and more inclusive definition of adolescence is essential for appropriate framing of laws and service systems.’

Latest figures show that the average age for adults to get married has risen by eight years since the 1970s. It was 32.5 for men and 30.6 for women in 2013 – compared to 24.9 years and 22.9 years respectively in 1972. 

Professor Russell Viner, president-elect of the Royal College of Paediatrics and Child Health, supported the scientists’ calls to expand the definition of adolescence, saying: ‘The average age for leaving home is now around 25 years for both men and women.’

But Dr Jan Macvarish, a parenting sociologist at the University of Kent, said there was a risk of ‘infantilising’ young adults.