Napo, a trade union and professional association that campaigns on behalf of probation and family court staff said youth offending teams have proven to be "very successful" at reducing reoffending rates.
"Extending them to work with [18- to 21-year-olds] would enable a smooth transition to probation, reduce reoffending and reduce the number going into custody," the organisation's manifesto for the general election states.
Calls have previously been made for the move by Labour.
In 2014, then shadow justice secretary Sadiq Khan said the change could help reduce the numbers of 18- to 20-year-olds coming into contact with the justice system and being imprisoned, replicating improvements seen with under-18s.
Rob Allen, a youth justice consultant, said youth justice has "offered a ray of light in penal policy" through big reductions in numbers on court and custody in recent years.
"Now's the time to extend the successful leadership of the Youth Justice Board and the multi-agency approach of youth offending teams to the young adult age group of 18- to 21-year-olds."
The Napo manifesto also calls for action to be taken to address the "workload crisis".
"Workloads in both probation and family court are dangerously high," the manifesto states.
"In Cafcass the workload has significantly increased causing stress and poor health to our members. It has a direct impact on the quality of work delivered and places the public at risk of harm."