Who says soldiering is a young man's game? Staff Sergeant Eric Ferguson joined the US army in 1973 and -- now approaching his 60th birthday -- he is still on active duty, leading distribution convoys through some of the most dangerous parts of Afghanistan.

Armed with an M4 machine gun, Ferguson supervises trucks as they inch their way across the eastern province of Khost delivering ammunition, rations and maintenance supplies to troops stationed in remote combat outposts.

Roadside bombs are a constant threat. One exploded just ahead of his most recent convoy, causing a nerve-wracking four-hour wait as specialist route clearance teams arrived to find a safe way ahead.

But for Ferguson, the life of a soldier is uniquely rewarding and entirely suitable for a man who will turn 60 on January 14.

"I enjoy the camaraderie and I love my soldiers," he told AFP. "They make you laugh at the things they say.

"They are mostly in their 20s and they are often surprised when they hear how old I am, but they soon realise I am just one of the guys."

Ferguson, who shows few signs of his veteran status besides flecks of grey in his clipped moustache, goes to the gym every morning and also turns out for his platoon basketball team.

"Playing with younger guys, it wears you out. They don't give me a break, they're hard," he laughs. "I passed my medical no problem last month, and I'm blessed to be as fit and healthy as I am."

The 39 men and three woman in his CLIP (combat logistics patrol) platoon gently tease him about his age, and have nicknamed him "Old Gravy Bones" -- which he has written on the door of his tiny office.

"I don't know, I guess it means I'm old and strong," he says with a shrug.

Ferguson expected to stay in the army for just three years when he signed up 38 years ago, but found he came to like military life more and more.

Then he served as a part-time reservist in the national guard for many years before re-joining full-time in 2008 and he will finally hang up his uniform in June.

"You have to leave before you turn 62, and next year I will have done the 20 years of active service that mean you get paid a pension and other benefits," he said.

It will be a well-earned reward for Ferguson, who served in Germany, Hungary and Iraq before his two tours of Afghanistan.

Only last year, while deployed in the eastern Afghan city of Jalalabad, he was using his machine gun in battle to beat off insurgent ambushes and firefights.

"We've had a couple of scares but haven't had any injuries so far this time," he says of his current deployment based at Camp Salerno in Khost province, where US troops battle with Taliban rebels.

"I could chose to stay here on the base, but I think I should be out with my soldiers most days, even if we have to spend 12 hours or more on the road," he said, sitting next to his 20-pound (10 kilogram) flak jacket.

Ferguson, who has four children aged between 25 and 33, finds that being four decades older than the youngest soldiers means some naturally gravitate to him for advice.

"Because of the experience I have, I talk to them about their plans to marry, about divorces, and whether to stay in army. This generation is marrying young and divorcing young, there's a lot of that going on."

It is a mentoring role that he knows well, as he is also the lay preacher at Salerno's evangelical gospel service every Sunday.

"I get pretty fired up when I preach," he admits. "The church is strong in the army because being away from home brings people together."

As well as his religion, Ferguson credits his family for giving him the support to cope with the pressures and dangers of war.

"They want me to go home, but they know this is my life and if something happened to me they know I would die doing something that I like doing. We have spoken about that," he said.

"I'm lucky. My wife is the most wonderful woman I have ever known."

Ferguson says the two biggest changes he has seen in the army have been modern technology and a less deferential attitude among junior soldiers.

"The army was more disciplined. Back in the day, soldiers didn't question authority as they do today," he said. "And the tempo has changed with communications and all the new equipment."

He says he will look back with pride when he finally signs off next year -- but his "retirement" may not provide much time to reminisce.

"I worked as a trained embalmer at a funeral home when I was in the reserves," he said. "Now my ambition is to set up and run my own funeral home business."