Vietnam War infantry conscripts were often only 19, as Redgum's song reminds us.
But a group of boys who enlisted in the navy around the same time were even younger - some aged just 15 years and 9 months when they signed up, voluntarily, to be sent to HMAS Leeuwin in Fremantle for training.
That was 50 years ago, and the survivors of the 8th Intake at Leeuwin will have the honour of leading this year's Anzac Day march through the streets of Wollongong.
They were boys when they joined up, from all around the country. A little over a year later, while they had been hardened by the brutal training, they were still boys as they steamed towards a war zone on HMAS Sydney and HMAS Melbourne.
Woonona man Bob Green remembers sailing on the Sydney as it delivered the first Australian infantrymen - the 1st Battalion, Royal Australian Regiment - to the conflict in Vietnam.
"I wasn't even 17½," he told the Mercury.
"I didn't even have overnight leave.
"I couldn't go into a pub; I couldn't vote.
"But I went into a war zone. You think, that wouldn't happen today."
The Sydney would become known as the "Vung Tau ferry", for its role as a fast transport to get troops to the port city Vung Tau.
From there the conscripts would take a 30-kilometre drive north along Route 2, to the taskforce base at Nui Dat. Long Tan, where Australia would lose 18 men in a muddy plantation one evening in 1966, was five kilometres to the east of the base.
On the Sydney's armoured escort, the aircraft carrier HMAS Melbourne, was Richard Watkins, who now lives in Albion Park, but who 50 years ago crossed the country from Ipswich to train at Leeuwin.
He was 15 and nine months - the age at which boys these days get their learner's permit, earn a few dollars collecting trolleys, or fill their heads with video games, Facebook and flirting.
Instead of a 17th birthday with school friends, he celebrated with his shipmates in Singapore.
Mr Watkins said he joined up because it seemed like a good idea - a way to find adventure, and get out of school.
At least, that's what he figures in hindsight.
"I don't think you think too much at that age; you're really young," he said.
He became a helicopter mechanic on the Melbourne. Both he and Mr Green served in the navy for 12 years - which at the time was the minimum you could serve, once joined up.
Five decades later and the boys of HMAS Leeuwin are holding their 50th anniversary reunion in Wollongong this weekend.
It's the fifth reunion they have held, every five years, a different city each time. More than 60 veterans are expected this weekend, with events centred on Wollongong Golf Club.
It wasn't easy for the boys at Leeuwin. The training and the treatment was hard, not the type of childhood that is commonly thought of these days as a right.
New recruits were treated badly - "Everything you hear is true," said one of the men.
"It was hard, they were very hard on us," Mr Green said.
"Leeuwin was hard. There was a bit of bastardisation, that's been well documented. But if you could survive Leeuwin, you could survive anything the navy could throw at you."
Fremantle is a long way from Wollongong, or Ipswich, and the trip to get home for leave took seven days for the young Mr Watkins. But he would be happy to be home for a few days.
"You got treated terribly, you really did," he said.
"It makes you a stronger person - it hasn't made any of us worse."
Stronger, then, they made their way in the navy, some serving 40 years, reaching ranks as high as captain.
Mr Green was one of those on board the Melbourne - an aircraft carrier - when the US Navy's destroyer USS Frank E. Evans turned into its path and exploded, sinking into the South China Sea, taking 74 US sailors with it.
Of Leeuwin's 209-strong 8th Intake class, 23 have since died, many of them not from combat but from diseases associated with being on ships.
There was significant asbestos exposure on many ships, often with deadly consequences. And water picked up in Vietnam, distilled for drinking, was later found to contain Agent Orange, which was not removed by the distillation process.
Those are the scars, but many of the men getting together in Wollongong this weekend will remember their youth in the navy fondly.
"A lot of people will tell you it's different - but I have only good memories of it, of Leeuwin and the navy."
Now 66, Mr Green agrees.
"I met some great blokes and I've got the best friends a man can have," he said.
"A friendship formed in the service is a lifelong friendship."
Correction: An earlier version of this story incorrectly referred to Richard Watkins as Richard Franklin.