A nine-year-old US boy has become the youngest person in recorded history to reach the summit of Argentina’s Aconcagua mountain, which at 6962 metres is the tallest peak in the western and southern hemispheres.

Tyler Armstrong of Yorba Linda, California, reached the summit on Christmas Eve with his father Kevin and a Tibetan sherpa, Lhawang Dhondup, who has climbed Mt Everest multiple times.

They were in fine spirits on Friday as they left Aconcagua, whose sheer precipices and bitter cold have claimed more than 100 climbers’ lives.

“You can really see the world’s atmosphere up there. All the clouds are under you, and it’s really cold,” Tyler said, describing the summit to The Associated Press.

“It doesn’t look anything like a kid’s drawing of a mountain. It’s probably as big as a house at the summit, and then it’s a sheer drop.”

Only 30 per cent of the 7000 people who obtain permits to climb Aconcagua each year make the summit, said Nicolas Garcia, who handled their logistics from down below.

No one under 14 is usually allowed, so the family had to persuade an Argentine judge that Tyler could safely accomplish the feat.

In their case, they took the “Polish Glacier” route, which doesn’t require climbing, and roped themselves together only when crossing steep ice-covered slopes.

“Any kid can really do this, all they have to do is try. And set their mind to the goal,” said Tyler, who worked out twice a day for a year and a half to prepare for the climb.

He also held fundraisers, not only to defray the cost but to raise money for CureDuchenne, which funds muscular dystrophy research.

“I think Tyler’s record speaks for itself and because I think he’s doing it for a good cause, he’s doing it to help other people, I think the judge recognised that,” said his father, an emergency medical technician.

Tyler’s mother is a pediatric neuropsychologist, and they also have another son, Tyler’s younger brother Dylan.

“Most people think we as parents are pushing Tyler to do this, when it’s completely the opposite. I wouldn’t climb it if I didn’t have to, but my wife makes me do it to keep watch on him,” he said.

“He’s a great dad,” Tyler said. “At 20,000 feet, he wanted to turn around but I kept him going. And the day we were getting off the mountain, he had a blister and it popped … He made it to the summit and everything but that dang blister made him ride a mule.”

Aconcagua’s previous record-holder was Matthew Moniz of Boulder, Colorado, who was 10 when he reached the summit in 2008.

There was one younger boy who climbed the lower slopes of Aconcagua, Garcia noted: an Inca boy was sacrificed some 500 years ago at 5300 metres on Piramide, one of the mountain’s lower peaks.

Scientific tests on the mummy, recovered in 1985, put his age at about 7.