Vulnerable children should be allowed to stay in care until they are 25, the children's commissioner for England has said.
Nearly a third of young people who had to leave residential care at 18 or foster care at 21 felt they had been forced to fend for themselves too early, Anne Longfield said.
The survey of nearly 3,000 children and young adults also found more than half did not understand why they were no longer living with their parents.
With around 69,000 children in care in the UK, Ms Longfield said it was "unacceptable" that about 35,000 youngsters were confused about why they were in care.
She told the Press Association: "This group of young people are probably the most vulnerable, often having suffered from extreme neglect, often abuse and often trauma.
"They are no longer living with their parents and have often not had stable placements. They are the least likely to find employment and most at risk of drugs, alcohol and crime.
"They need support as they go into their adult life."
Around 10,000 young people aged 16-18 leave care each year, according to the Who Cares Trust.
Ms Longfield, whose role is to protect the interests of children in England, said more investment was needed in the care system to ensure support can be given up to the age of 25.
She said: "When a child reaches 18, a parent would not wave goodbye to them for good and close the door to them, so we shouldn't do so for children in care who, more than any of us, need a positive springboard for the future."
Ms Longfield said children in care should be able to keep the same social worker for longer and receive therapeutic care to " recover from their difficult past experiences".
She added: "We know that many children who leave care still struggle to reach their full potential and helping them to overcome their harmful experiences and build their resilience and emotional well-being for the future will help them to do so."
The survey by the University of Nottingham also found nearly two in five (39.6%) children in care had been moved up to three times in the last two years. One in 10 had been moved at least four times.
A Department for Education spokeswoman said: "We are committed to improving the lives of care leavers. That is why we have introduced a comprehensive series of reforms since 2010 to help and support them.
"We welcome the survey's findings that the vast majority feel they live in the right place, are treated with respect and feel part of the family.
"But we are not complacent and will continue to work to make sure all young people, whatever their background, have the best opportunity to achieve their full potential."
Barnardo's chief executive Javed Khan, backed Ms Longfield's call.
He said: "Children who leave care have often had problems to deal with in their lives that most people couldn't even imagine.
"Working with more than 2,000 care leavers in England Barnardo's knows just how much they can need a helping hand. That's why we continue to campaign for the age of support to be raised to 25.
"Why shouldn't those who were in care get support that mirrors what parents give to their children well into early adulthood and beyond?
"These changes would need to be properly resourced. Strong and consistent support sends a clear message to these young people that Britain is investing in their future."