The number of young heroin addicts in the UK has dropped to its lowest recorded level.
Some 4,268 adults aged 18 to 24 started treatment for heroin addiction in 2011/12 for the first time, a fall of nearly two-thirds from 11,309 in 2005/06, figures from the National Treatment Agency for Substance Misuse (NTA) showed.
But the over-40s now make up almost a third of the whole treatment population, with more than 16,000 starting a new course of treatment last year.
Heroin also remains the biggest problem for drug addicts, with four in five of the 197,110 adults in treatment in England being treated for heroin dependency or for heroin and crack.
Paul Hayes, the NTA's chief executive, said: "Treatment needs to accelerate its recovery focus if more of the ageing heroin population are to successfully complete treatment and get their lives back on track.
"And we should be mindful that economic problems have historically exacerbated drug addiction."
More over-40s are also dying from drug misuse, up to 802 last year from 504 in 2001, the figures showed.
The growing number of drug addicts over 40 is fuelled by an ageing population, rather than by new users, the NTA said.
Of all those in treatment, 31% were receiving help for heroin or heroin and crack, the figures showed.
A total of 96,343 were receiving help for heroin dependency while a further 63,199 were being treated for heroin and crack use.
The NTA report also showed that 29% of the entire treatment population in the last seven years successfully completed their treatment and did not return.
Hayes said there was was a "savvier generation of young people who seem to know what heroin and crack are going to do to you".
"The original pool of heroin and crack addicts is shrinking," he said. "And because fewer people are using heroin or crack, it's not being topped up.
"Less positively, there's an increasing challenge from older drug users, many of whom are still in the system and others who began using in the 80s and 90s are now beginning to access treatment as their health deteriorates.
"These people became addicted to heroin during the epidemics of the 80s and 90s. Some are trying treatment for the first time this year, others have tried treatment, relapsed and then come back in, or have entrenched use that presents a significant issue for treatment services, not least because of the sheer numbers in treatment who are aged over 40."
He went on: "There are risks ahead. No one could have predicted in the 1980s that one of the consequences of the recession would have been mass heroin use. It came from nowhere.
"Whilst this recession has not produced the same levels of youth unemployment that the 1980s did, youth unemployment, hopelessness among young people, provides fertile territory for the next drugs threat to take hold.
"Young people who are at risk of unemployment, whose prospects are limited, are much more likely to become addicted than people who have a stake in society and can actually see themselves making progress long term."
The continuing investment in drugs treatment "cannot be guaranteed in the current financial climate", Hayes added.
With budgets moving to local authorities from April next year, "there is a risk that squeezed local authorities will disinvest, not necessarily from direct treatment provision, but from services that support drug users' long-term recovery.
"That will then impact on the likelihood of people being able to sustain the benefits that they've accrued in treatment."
Hayes added: "There's no evidence of swathes of people in their 40s and 50s beginning to use heroin and crack as they get older.
"It's a population that began using 20 or 30 years ago."
Clinicians offering treatment also need to "up their game" to ensure people are not being kept on methadone scripts for longer than they need to be.
"Methadone is an absolutely crucial first step for many people with heroin misuse problems," he said.
"However, it's also true that too much practice in the past has been over-reliant on methadone and hasn't been ambitious enough to make sure that we use it as a platform for recovery, rather than people remaining safe, secure, but not being given every opportunity to actually leave addiction and treatment behind."
Around 140,000 addicts are on some form of prescription treatment, figures show.
Asked if he agreed with former justice secretary Ken Clarke that the war on drugs was lost, Hayes said fewer people were using drugs, fewer people were dying from drugs, more people had access to treatment, more people were getting better and drug-related crime was down.
"We are having progress," he said.
"If by winning the war on drugs you mean 'are there still some people using drugs and are there still harms flowing from drug use?', clearly - but if that's your criteria for winning the war on drugs, then so be it, we're not winning the war on drugs."
But he rejected suggestions that the Government should do a "hand-brake turn" and start legalising or decriminalising drugs.
"My view is emphatically no," he said.
"Any move towards legalisation would certainly mean an increase in use, particularly in the most problematic drugs.
"It isn't (Sir) Richard Branson having an extra spliff that's an issue.
"It's people who struggle with life in Liverpool and Manchester and the East End of London who are actually going to bear the consequences."
Martin Barnes, chief executive of DrugScope, said the decline in heroin use and dependency among young adults was "perhaps a sign that drug treatment has been turning the tide of the heroin epidemic of the 1980s".
"We are, however, aware of the rising numbers of people over 40 in the treatment system, who often have particular needs around long-term and entrenched heroin use," he said.
"In difficult economic times, there is a strong and compelling case for national and local investment in drug and alcohol treatment."