There are also biological factors to consider, including the fact that the brain and body both continue to develop after 18. In 2010, The New York Times published a much-read article about how the experience of being a 20-something is changing. For instance, the brain doesn't stop its development at the age of 19. Many people's wisdom teeth don't come through until the age of 25. This is nearly an eight-year increase since 1973.
Lead author Proffessor Susan Sawyer, director of the centre for adolescent health at the Royal Children's Hospital in Melbourne, wrote, "Although many adult legal privileges start at age 18 years, the adoption of adult roles and responsibilities generally occurs later".
She explains that these delays mean "semi-dependency" that is characteristic of adolescence.
The average age of the onset of puberty has been steadily dropping in the developed world for decades, due to improvements in health and nutrition.
Sawyer said the current definition of adolescence is "overly restricted". Still, other laws are all over the place - people get their driver's license at 16, but can't rent a auto until age 25.
And although this letter argues that adolescence still starts at 10, they think the ripe old age of 24 better reflects "adolescent growth and popular understandings of this life phase".
Dr Jan Macarvish, University of Kent Sociologist told the BBC that the paper's suggestions may may further "infantalise" young people and we should "continue to have the highest possible expectations of the next generation".
However, not all experts agree with a new definition since it could help "further infantilize young people".
We should not risk "pathologising their desire for independence".