Effects of autism 'long-lasting'
The NAS says autistic teenagers need more support
And just 15% of them are in full-time employment, says the society's "Moving on Up?" report.
But the society says this could improve if the right planning and support were offered to young people with autism.
England's Children's Commissioner, Sir Al Aynsley-Green, said the government had made progress but work was still needed to help young autistic people.
The NAS is calling for better support and services for young people with autism during the critical "transition stages" between school, higher education and employment.
Change can be difficult for young people with autism to cope with, and the transition from childhood to adult life can be especially problematic.
The government's Special Educational Needs Code of Practice says all children with a statement of special educational needs should have transition planning from the age of 14.
Yet the NAS found that only about half, 53%, of young people receive such plans during their education, and only one third of those in mainstream schools do so.
Rosemarie Mason has five children, three of whom have been diagnosed with childhood autism - twins Sean and Eoin who are now 19, and 17-year old Mehal.
She said: "Like any parent I want them to do what they want to do, to support themselves and to make decisions for themselves.
"I'm realistic enough to know they're going to need support - they're going to need a network of support."
"But that isn't what's designed for adults with disabilities."
And Deborah Packenham, whose son Ieuan also has autism, said: "It's very scary because I know that when he reaches a certain age he will no longer be entitled to the support he's entitled to as a child.
"He'll be an adult, and support is very patchy."
NAS head of policy Amanda Batten said many young people with autism were failing to fulfil their potential due to a lack of appropriate support at the vital transitional stage in their lives.
She said: "It is imperative that there is early and effective transition planning for every young person with autism.
"Many have a great deal to offer and should have access to appropriate support.
"If transition fails, young people can find themselves embedded more firmly than ever in the family home, increasing stress on the family and resulting in more isolated lives."
Sir Al Aynsley-Green, Children's Commissioner for England, said: "This research shows clearly why we must be extra vigilant to the needs of young people with autism so that they receive the right financial and emotional support to cope with adult life.
"Recent government initiatives for families with disabled children are welcome, but I continue to be told that poorly co-ordinated services and a lack of adequate resources to help them navigate their way through are adding to their difficulties - this must be addressed as a matter of utmost urgency."
A spokesman for the Department of Children, Schools and Families said it was spending £19m on a transition support programme to give disabled young people more "choice and control" as they move into adulthood.
"We are developing an autism pack for schools, which will include information on good transition planning," he added.http://news.bbc.co.uk/1/hi/health/6897908.stm