Care leavers should receive support according to their needs rather than their age or whether they are in education or training, MPs have heard.
England's Department for Education permanent secretary Chris Wormald told the Commons Public Accounts Committee those in education or training were entitled to support until they were 25.
But for other care leavers, this entitlement ended at 21.
And this "anomaly" meant the most vulnerable had the least support.
"This was done for the best of reasons," Mr Wormald told the committee.
But he added: "As soon as you start setting these entitlements in law we end up with these age cut-offs and therefore a cliff-edge."
Mr Wormald called for local authorities to be given the flexibility to decide how long to continue to support care leavers.
About 10,000 young people aged 16 to 18 leave care each year in England, according to the National Audit Office.
And among 19-year-old care leavers, 41% were not in employment, education or training in 2013-14, compared with 15% of the age-group overall.
Care leaver Emanuel welcomed the government's Staying Put policy, which allows young people to stay with foster parents after the age of 18, but said the scheme had not worked well for him because of poor advice about funding.
The committee also heard from Dembo, who said poor advice after leaving his children's home had resulted in periods not in any kind of work, training or education.
"It is disgraceful that at 21 years old that if someone has been looked after by the local authority and they are not in education, employment or training and we close their case and those who are in education get support until they are 25," he said.
"It means the system is not working... that's when we need to support them more.
"If we have children we are not going to kick them out of our house at 21 years old if they are not in education, employment or training.
"They need to start fixing that one."
The government says it is committed to improving the lives of care leavers.