Young people continuing their education for longer, as well as delayed marriage and parenthood, has redefined common perceptions of when adulthood starts, scientists wrote in the Lancet Child & Adolescent Health journal.

Societal and biological changes have prolonged the adolescent years from the ages of 10-19 to now ending at 24 years old, scientists said, leading to a debate about whether new policies would benefit or “infantilize” the older adolescents.

Puberty used to occur around age 14, but now has lowered to the age of 10 because of improved health and nutrition in much of the developed world, the BBC reported.

"Although many adult legal privileges start at age 18, the adoption of adult roles and responsibilities generally occurs later." — Prof. Susan Sawyer

As a result, in the past 150 years the average age for a girl's first menstruation has become younger by four years in industrialized countries like the U.K.

Young people are also getting married and having children later, the report said. In 2013, the average age for a man to enter their first marriage was 32.5 years and 30.6 years for women across England and Wales, the report said, citing U.K.'s Office for National Statistics, which represented an increase of almost eight years since 1973.

Other biological arguments for why the definition of adolescence should be extended include that the body continues to develop, the channel reported. For example, the brain continues to mature past the age of 20, working more efficiently.

Professor Susan Sawyer, director of the center for adolescent health at the Royal Children's Hospital in Melbourne, calls the dynamic “semi-dependency.”

"Although many adult legal privileges start at age 18, the adoption of adult roles and responsibilities generally occurs later," she said.

Sawyer said the current definition of adolescence is “overly restricted.” She told the outlet that this social change needs to inform policy, including extending youth support services until the age of 25.

"The ages of 10-24 years are a better fit with the development of adolescents nowadays,” she said.