Adolescence now lasts from the ages of 10 to 24, although it used to be thought to end at 19, scientists say.
Young people continuing their education for longer, as well as delayed marriage and parenthood, has pushed back popular perceptions of when adulthood begins.
Changing the definition is vital to ensure laws stay appropriate, they write in an opinion piece in the Lancet Child & Adolescent Health journal.
But another expert warns doing so risks “further infantilising young people”.
Puberty is considered to start when the part of the brain known as the hypothalamus starts releasing a hormone that activates the body’s pituitary and gonadal glands.
This used to happen around the age of 14 but has dropped with improved health and nutrition in much of the developed world to around the age of 10.
As a consequence, in industrialised countries such as the UK the average age for a girl’s first menstruation has dropped by four years in the past 150 years.
Half of all females now have their period by 12 or 13 years of age.