UPDATED 12.51PM: Social development agencies are teaming up to try to keep young people in state care until they're 21.

A scathing review of Child, Youth and Family recommends a new advocacy service for children in state care - run by the philanthropic sector.

Social Development Minister Anne Tolley has announced the service is in for a shake up, aimed at focusing less on administrative procedures and more on the children in its care.

Social development agency Lifewise said one of the first changes that needs to be made is increasing the age at which young people can leave care from 17 to 21.

Lifewise general manager Moira Lawler said 17 is just too young to be left alone – at that age young people can't even sign a lease and many young people end up on the streets.

She said the average age for a New Zealander to leave home is actually 23.

“If it’s good enough for mine and your children to receive support until they’re 23, why are we saying that children who have CYF’s experiences should lose that support at 17,” she asked.

Ms Lawler believes this increase will be a money saving move for the country in the long term.

“It’s much more effective to provide them the support to make sure that they manage to settle well than it is to meet their needs when they end up in mental health and addiction systems or are homeless and needing intensive housing support.”

Otago University associate professor Nicola Atwool says stepping in earlier for "children [who] often having quite long exposure who finally coming into care with very high levels of need that then haven't been responded to appropriately."

Anya Satyanand, of youth social work advocacy group Ara Taiohi, believes the young people should be at the heart and the design of the service.

"The most powerful thing that the panel could do at this point is, in terms of the operating model, honour what is laid out in the report about young people needing to have the right to self-determine."