Adulthood is often defined as being fully grown and able to stand on your own two feet but for many people, dependence on their parents is extending well into their 30s or even later, with one expert saying we may be experiencing childhood until we are 40.
University College London Anthropologist Dr Brenna Hassett said we are lengthening our childhood - staying children for longer.
She said the switch from childhood to adulthood is when people stop investing resources in themselves and invest in other people by making contributions to society such as through a career or having children.
"We as a species seem to be pushing that date later and later," Dr Hassett said.
In her book Growing Up Human: The Evolution of Childhood, Dr Hassett says her childhood effectively ended when she had her first child at age 39.
She said nowadays many people still rely on their parents until they are 40, making it harder to see themselves as a grownup.
"It's not so easy to see yourself as a grownup like you know our parent's generation. It's hard to buy a house, establishing a career sounds like a dream."
It has become the new normal for people to train for careers longer, with many people seeking higher education, whereas earlier generations left school a lot younger.
"Kids go to university, kids are in training and apprenticeships for longer and longer now. But it does mean that you're not quite grown-up perhaps," Dr Hassett said.
She said we have pushed the boundary on the period we are dependent, lengthening our childhood
She said while we have extended our lifespan, what humans have seemed to have done is start to grow slower.
"We have these really long life spans… But we start to grow slower and slower," Dr Hassett said. "We really pushed the boundary on this period where we are dependant, where society is still investing on us."
She said a lot of people are happy to be labelled potentially a child at 40.
"People seem to be really keen to point out that they are never growing up."