There has been a surge in older children and teenagers with "complex needs" entering care in England.
The Children's Commissioner has warned they experience more instability, and are often "at risk of exploitation".
One care leaver told the BBC he felt "vulnerable" after moving between 20 different foster carers.
A new study found the number of those aged 16 or over in care increased by 25% from 2013/4 to 2017/18, while those over 13 rose by 21% in the same period.
Nearly a quarter (23%) of children in care are aged over 16, and 39% are in the 10 and 15 age range, according to the Children's Commissioner's 2019 Stability Index.
Anne Longfield, children's commissioner for England, said the care system was under strain because rising number of teenagers were more likely to have experienced issues that required specialist support.
The government said it has created a £2m fund to address some of the issues.
Teenagers are entering care due to issues including criminal or sexual exploitation, going missing from home or failing to get protection from their parents, the study found.
Increasing demand from older children has caused councils to spend more on individuals with acute needs, it suggested.
It also warned that older children in care experience higher levels of instability, including moving two or more times within a year.
'You feel vulnerable'
Orion Blake, 22, had more than 20 foster carers and 20 social workers growing up.
He told the BBC's Victoria Derbyshire programme that the experience had been "really difficult".
"It affects communication, really," he said. "You never know who you can talk to, who's there to support you and who's going to actually stay around.
"Emotionally, you feel vulnerable because you don't know who you can turn to."
Harmony, a 22-year-old care leaver who entered care aged 15, said children in her situation often go through a series of different social workers.
"If you're coming from an experience where you're having social worker after social worker it does really have an impact on your ability to see your worth to them," she said.
"You might internalise people moving on as something to do with yourself."
Maintaining relationships between child and carer are "key", according to Libby Thornfield, president of the Fostering Network.
"It will enable that child to understand how to develop relationships in the future," she said.
But Ms Longfield said older children in care are likely to move between placements because the system has not adapted to provide specialist support for their growing numbers.
"The profile of children in care has traditionally been much younger children," Ms Longfield explained.
"[Older children] are more likely to have complex needs, need more specialist support, and are much more likely to move between different placements because that support isn't appropriate.
"That means that those young people who are at very high risk don't have the support and stability that they need to be able to settle and build their future."
Ms Longfield warned that the current situation was "unsustainable", adding that children "are being denied the chance to put down roots, to feel part of a family and to settle at school".
"It is not surprising that they are often the ones most at risk of exploitation," she said.
A Department for Education spokesperson said: "Every young person should have as secure and stable an upbringing as possible - regardless of their circumstances or background.
"We have established a new national programme, backed by up £2m funding, to help local areas protect young people from being taken advantage of by criminals or 'county lines' gangs.
"We are also working to reduce the number of 'out of area' placements for children in care and to improve our response for dealing with missing people."