Longfield claims that the increasing proportion of teenagers entering the system over the last five years is contributing to high rates of instability experienced by looked-after children by local authorities across the country.
This is because older children are more likely to experience instability, such as a placement move, or a school or social worker change.
This year the index shows that the number of teenagers in care rose by 21 per cent between 2012/13 and 2017/18, while the rate for the youngest children aged from birth to five, fell by 15 per cent.
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The number of over-16s entering care in the same period rose by 25 per cent, meaning around one in four children in care are over 16.
Longfield states: "The new norm is shifting so that fewer babies and very young children are being taken off parents who cannot cope.
"Instead it is teenagers who are being taken into care because they are experiencing issues such as criminal or sexual exploitation, going missing from home, and parents being unable to protect them."
According to Longfield, they are significantly more likely to have the following issues flagged up by social workers:
- Child sexual exploitation (six times more likely)
- Going missing from home (seven times more likely)
- Gangs (five times more likely)
- Trafficking (12 times more likely)
- Child drug misuse (four times more likely)
And as many as 20 local authorities had 80 per cent or more looked-after children who had experienced a school, social worker or placement move in a year.
These include: Rutland at 91 per cent, Sutton at 90 per cent, Southend-on-Sea at 89 per cent, Cambridgeshire at 88 per cent, Luton at 87 per cent, Sandwell at 86 per cent, and Wolverhampton at 85 per cent.
Nationally, more than 45,000 children in care - three in five - experienced at least one change in social worker in 2017/18, while more than 20,000 - one in four - experienced two or more changes.
The proportion of children in care experiencing multiple placement moves in 2017/18 ranged from 4 per cent to 20 per cent across local authorities, while the proportion of children experiencing a mid-year school change ranged similarly from 4 per cent to 22 per cent.
For switching social workers the variation is even greater: from zero per cent to 51 per cent between local authorities.
In local authorities with higher rates of agency staff, higher rates of social worker turnover and higher social worker vacancy rates, children in care are automatically more likely to experience multiple changes of social worker in a year.
Longfield described some teenagers as "pin-balling" around the care system. She added: "Often they have the most complex and expensive needs. In one local authority 20 per cent of the entire budget is being spent on just 10 children. This is completely unsustainable."
Reacting to the index, Eleanor Briggs, head of policy and research at Action for Children, said: "This is yet more evidence of a care system that's failing some of our most vulnerable children rather than giving them the safe and happy childhood they need."
Briggs added that the charity believes that the repeated changes for some children had an "awful impact" because "they have to change homes, families, friends, social workers and schools, sometimes over and over again".
She said: "The care system should be there to support the recovery of these vulnerable young people, not cause them upheaval, loss and distress.
"The new government must urgently address the root causes of this problem, providing children with the right home and the right support, as well as properly funding early help services which work with families before problems reach crisis point."
The Independent Children's Homes Association said that it was necessary for the government to fund a 15-year workforce strategy to stabilise services and in turn, "create stability for children and young people".