Elements of the youth justice system could be extended to cover criminals up the age of 21 under plans being considered by Labour.
The current system in England and Wales only covers offenders up to the age of 18, but Labour will consider extending the work done by the Youth Justice Board (YJB) and local Youth Offending Teams (Yots).
Shadow justice secretary Sadiq Khan will say that replicating the success of the youth system would cut crime and reduce the number of people in prison, saving taxpayers money.
In a speech to the Reform think-tank he will say Labour's shake-up of the youth justice system in the late 1990s had led to "stunning falls in first-time offending and the numbers behind bars have halved".
"There's so few young people in prison today that young offending institutions have closed down and the taxpayer has saved tens of millions of pounds. This is a double success that's so far eluded the adult justice system."
Under the proposals being considered should Labour win the 2015 election, the YJB's remit would be extended from 18 to 21 and locally-run Yots would take on the additional responsibility of 18, 19 and 20-year-olds.
Mr Khan will say: "This is a sensible step, building on what works, extending it to a decent chunk of the most prolific offenders. And, what's more, working with another important group of first time and repeat offenders."
Labour stressed that the idea would only be implemented if the plans were workable and affordable, and insisted that it was not aimed at downplaying serious crimes or reclassifying people aged 18 to 21 as youths.
The shadow justice secretary will say: "It's a vote of confidence in locally led work to reduce re-offending and the numbers in custody.
"We will explore the workability and costs, but let me be clear: I believe this is an idea whose time has come.
"If we can recreate even a fraction of the success of the youth justice system, we'd cut crime, cut prison numbers and save the taxpayer money."
He will also indicate the possibility of devolving youth custody budgets to local areas.
"Devolving the budget for youth custody down to the local level could create a system that rewarded the agencies involved so that reductions in custody levels delivered a financial saving to local authorities.
"This would build on the current system whereby local authorities are now obliged to stump up the cost of custody for those on remand, which has incentivised them to do something to prevent this happening in the first place.
"It is a promising idea and one that we will certainly look at the feasibility of piloting should we win in 2015."
He will attack Justice Secretary Chris Grayling's £85 million secure college as a "vanity project" and a "teenage titan prison".
Mr Khan will also dismiss the Government's privatisation of probation services as "evidence-free, ideologically-driven codswallop".