Young people are now an endangered species in the Australian Public Service, with outsourcing and changing hiring policies to blame, according to a public-sector employment expert.

Only 2.5 per cent of federal public servants are under 25, the latest figures show, and their numbers are shrinking fast - down from 5 per cent in 2007.

Under-25s represent more than 15 per cent of the total Australian workforce and the nation's latest youth unemployment rate is more than 27 per cent.

Meantime, the Commonwealth's 55-plus brigade is the only age group in the bureaucracy that is growing.

Public sector workforce expert Linda Colley, of the of Central Queensland University, says changes in hiring practices and policies have resulted in the public service abandoning its traditional role of training Australia's young workers.

According to the latest Statistical Bulletin from the Public Service Commission, some departments risk becoming "Dad's Armies", with nearly 60 per cent of Defence bureaucrats and nearly 55 per cent of Veterans' Affairs staffers over 45.

"Representation of young people, less than 25 years of age, decreased again this year," the bulletin noted.

"At June 2014, 2.5 per cent of all ongoing employees were in this age group - down from 3.1 per cent last year.

"This has been a consistent and steady trend. At June 2007, young people accounted for 5 per cent of all ongoing employees."

At the other end of the spectrum, the commission noted a very different story.

"The 60-and-over age group had the largest growth, 0.4 per cent, in ongoing employment this year, followed by the 55- to 59-year age group, increasing by 0.2 per cent," the bulletin read.

"All other age groups decreased in size.

"The proportion of employees 50 years of age and over has grown strongly over time, increasing from 20 per cent of all ongoing employees at June 2000 to 31.4 per cent at June 2014."

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The commission told The Canberra Times that there were no programs in place to combat these demographic workplace trends, and all Australian employers had to cope with an ageing workforce.

Dr Colley said there were "a heap of reasons" for the near extinction of young bureaucrats, and the public service had largely abandoned its role as a recruiter and trainer of young unskilled workers.

"In the old days, public servants used to be recruited from school so there was a bias in favour of youth employment," she said.

The the trend of departments abolishing training programs and cadet recruitment had accelerated since the global financial crisis, increasing the squeeze on jobs for young people in the APS.

The trend to outsource functions regarded as "non-core" activities was also identified as a potential killer of public service jobs for young people.

"Outsourcing usually doesn't happen with skilled policy jobs, it usually happens with unskilled lower level jobs," Dr Colley said.

"I think that outsourcing has very likely seen a lot of those lower-skilled, more mechanical jobs that are more likely to be inhabited by youth go."

She also noted that the drop in youth employment had coincided with a drop in the number of public servants on the lower classification bands and the phenomena known as "classification creep".

"There is a relationship between fewer jobs being offered at those lower levels and a reduction in youth employment," she said.

"That's the problem with open competitive systems: it's a good theory but it does leave some people behind."

The CQU academic called for a rethink of the bureaucracy's role as an employer, in light of the nation's soaring youth unemployment rate.

"You would think that with all the youth unemployment that we have, that there would be a refocus on using the public service as a training ground, it always used to be," Dr Colley said.

"They're very much focused on filling the position rather than looking at the labour market in general and saying 'we have a role in reducing youth unemployment by providing training'."