A higher age limit would make "a huge difference", she said, because young people could get on with their schooling.
"You're able to focus on that instead of worrying about where you're going to get the money, from where you're going to get your own food."
A study which laid the groundwork for the reforms found that 80 per cent of 21-year-olds who had been in care did not have NCEA Level 2.
"I'm surprised any of us could make it to NCEA Level 2," Tupua Urlich, 20, said.
"You've got to become an adult overnight on your 17th birthday. You're talking about us doing crime and all that - the financial system at 17 is not enough to sustain yourself."
Under the Government proposals, some young people would be given the option of staying in care until age 21, and transitional support would also be available until age 25.
Social Development Minister Anne Tolley said this new age limit was based on research which showed people's decision-making abilities continued developing until their mid-20s.
Announcing the reforms today, Ms Tolley said the care system needed to move away from crisis management and focus on long-term outcomes.
Young people should not have eight different placements while still in primary school, "with all of the associated trauma which comes from each move".
Ilene Tupaea-Mokaraka, 18, from Manurewa, said she had lived in nine different homes since being put into state care at age 7.
Mr Urlich, who moved into state care when he was 5, said he could not remember how many times he had moved homes.
"There's no question that the system that we have lacks a lot of stability in terms of placement, caregivers," he said.
"One of the big issues with that is relationships. You can't really develop them. The people you're supposed to build relationships with and trust to guide you and nurture you are constantly changing."
Another key part of the reforms was a dedicated, independent youth advocacy service to ensure children's voices were heard - both in the design of systems and in the day-to-day details such as placements.
Under the current system, Mr Urlich said: "It's like they're trying to figure out a crime but you're not talking to the victim."