Most new apprentices over 25, National Audit Office reports
More than two-thirds of the apprenticeships created in England in the past five years were taken by the over-25s, says a spending watchdog.
And one in five of those running in 2010-11 lasted just six months, the National Audit Office report said.
The watchdog says adult apprenticeships give a good return for public money, but warns that rapid expansion of the scheme brings risks.
The government says the report recognises that progress has been made.
With unemployment among young people in the UK rising above one million in the autumn, according to government figures, ministers are keen to help more people in to work.
Apprenticeships are open to the over-16s. They are mainly provided by private firms and further education colleges, who are funded by the government.
Employers are expected to contribute to the cost of the apprenticeship but the NAO says some are not doing so.
The number of apprenticeships doubled between 2006 and 2011 and the NAO office found that more people were completing their schemes, up from 47% to 75% in that time.
In 2010-11 a total of £1.2bn was spent on schemes. Nearly 443,000 apprenticeships were started in that time and just over 300,000 of them involved people over 19 (known as adult apprentices).
The NAO report also said apprentices over 25 accounted for 68% of the increase in new apprenticeships between 2006 and 2011.
Head of the National Audit Office Amyas Morse said: "The apprenticeships programme has been providing a good return for public spending. Nevertheless, the department should set its sights higher in order to get better value from the £0.5bn and rising now spent on adult apprenticeships each year.
"It needs to target resources more effectively; confirm the training provided is in addition to what would have been provided without public support; and make sure that the funding system is informed by robust information on the cost of delivery."
The report said apprenticeships gave a good return on public money, because people who had been on the schemes went on to earn higher wages, but said that the government could "improve value for money significantly, in particular by targeting resources more on areas where the greatest economic returns can be made".
Skills minister John Hayes announced in December 2011 that there would be a review into the quality and duration of all apprenticeships schemes, and MPs on the Commons' Committee of Business, Innovation and Skills are holding an inquiry, with public hearings due to start at the end of the month.
The NAO report revealed that 87 providers of short-term apprenticeship schemes are being investigated to see if they are meeting their contractual obligations.
In 2010-11, one in five apprenticeships (34,600) lasted less than six months, while 3% (6,200) lasted less than three.
The government recently said all apprenticeships for 16- to 18-year-olds should take a minimum of 12 months and that it might extend this to adult apprenticeships.
A recent investigation by the BBC's 5 live Investigates programme revealed how one scheme which has been axed promised to train teenagers as football coaches and was funded by £6m of taxpayers' money and left thousands of those taking part without a qualification.
Minister for Skills John Hayes said: "Unprecedented investment, backed by tough new measures to ensure that quality matches quantity, has helped make apprenticeships the gold standard vocational qualification.
"So I am delighted that the NAO has recognised the progress we have made and that they identify the extraordinary economic benefits of apprenticeships."
He added: "The report rightly identifies a need to prioritise investment where returns are greatest, and that is what the government is doing.
"We will continue to drive improvement by developing new higher level apprenticeships, giving training providers more freedom to meet the needs of local businesses, reducing bureaucracy and making financial incentives available to small firms hiring their first apprentices."