The Children's Society said that because there is no statutory requirement for councils to support children in need when they turn 18 they are often left without any help even though they remain vulnerable.
It said that there are currently around 58,000 children and young people aged 16 to 17 designated as children in need, who are in need of support but fall below the threshold for care proceedings.
However, the charity's report Crumbling Futures found that just three per cent of closed cases involving 16- and 17-year-old children in need are transferred to adult services for support.
Key areas of support, that drop off when they reach 18, cover issues such as child sexual exploitation (CSE), mental health problems, drug and alcohol abuse and domestic violence.
"Issues that young people referred to children's services as 16- and 17-year-olds experience include domestic violence, mental ill health, drug or alcohol abuse and a risk of CSE, and often a combination of these issues," states the report.
"In just over 50 per cent of cases of 16- and 17-year-olds referred to children's services for support, these issues are deemed serious enough by local authorities and young people are assessed as ‘children in need', recognising that without support from services the child's health and development may be compromised."
It adds: "Unfortunately, for many of these children the issues they struggle with are not going to improve or get resolved once they reach adulthood."
The Children's Society has called on government to broaden its review of children in need, which launched earlier this month, to include a focus on improving support into adulthood.
"While the review is focusing on improving how well children in need do in education, the charity wants it to look at all aspects of their lives where help is falling short," the Children's Society said.
Other recommendations include ensuring that children in need and child protection plans for 16- and 17-year-olds last until the age of 18.
The charity's report found that four in 10 child in need plans for the age group last for less than three months.
Councils should also be required to plan for young people's transition from children's services to adult services and take into account the possibility that support may be needed up to the age of 25.
"Approaching adulthood can be a difficult, awkward, time for many teenagers, but it can be even tougher if young people don't get the help they need to deal with serious issues in their lives," Children's Society chief executive Matthew Reed said.
"Help for vulnerable 16- and 17-year-olds who are not in care too often falls short then disappears from the age of 18 as they continue to struggle with issues including mental health, sexual exploitation, poverty and homelessness.
"The Children's Society wants to see better support for children in need as they prepare for adulthood and a comprehensive package of help after they turn 18 - with councils given the additional money they need to deliver this.
"Only then will more young people get the vital support they need to ensure problems arising from their childhood are addressed and do not blight their chances of thriving in the future."