He could ride wild horses aged six, drive a truck at 80mph at the age of eight and twice beat a European power-boat racing champion in competition at the age of nine. The Supreme Leader also climbed to the top of Mount Paektu, a snow-capped 9,000-foot volcano, without hiking gear.
These outlandish claims, told to the North Korean people, are documented in The Great Successor: The Secret Rise and Rule of Kim Jong Un.
The biography of Kim, by journalist Anna Fifield, also claims he wore a Colt .45 on his hip when he was 11.
Kim is also said to be a fan of Whitney Houston and “passed his days” as a child listening to the singer on a Walkman.
He was sent to live in Switzerland when he was 12 where he attended the Liebefeld Steinhölzli school near the city of Bern.
In 2009, aged 25, his status was elevated in North Korea.
Ms Fifield describes how propaganda created a “personality cult” around Kim.
She wrote: “The North Korean regime sent a booklet to every unit of the Korean People’s Army called The Material in Teaching the Greatness of Respected Comrade General Kim Jong Un.
“Among the supposed great feats it listed was that, at age three, Kim Jong Un would fire a gun and hit a light bulb a hundred yards away.
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“Another version of the story had him hitting a target ten times in ten seconds.
“By the time he was eight, he could not only drive a truck, but he could drive it at eighty miles an hour.
“Plus, he knew everything there was to know about the military, whether it be the army, the navy, or the air force.”
She goes on to describe separate propaganda which claimed Kim “could ride the wildest horses at age six, and that, when he was just nine, he had twice beaten a visiting European powerboat-racing champion”, driving at speeds of 125mph.
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The biography also describes how, on two occasions, North Korean state media reported Kim had climbed “through thick snow” to the top of Mount Paektu on the Chinese border.
Ms Fifield wrote: “As proof, the papers showed the overweight man on top of the nine-thousand-foot mountain – wearing a long woollen coat better suited for Pyongyang parades and black leather dress shoes.
“So great was the leader that he didn’t even require hiking gear to climb mountains.”
Dr Siegfried Hecker has visited the nation’s Yongbyon Nuclear Scientific Research Center, 90km to the north of Pyongyang, to assess its plutonium programme seven times.
The scientist, a former director of the US Los Alamos weapons laboratory and now a professor emeritus at Stanford University, has spoken to Express.co.uk.
He said: “My best estimate of North Korea’s nuclear program today is that it possesses sufficient fissile materials to produce approximately 45 nuclear weapons.
“The North has successfully conducted a sufficient number of short and medium-range rocket tests, when combined with the nuclear test history, makes it possible to reach all of South Korea and most of Japan with nuclear-tipped missiles.
“It has demonstrated ICBM-class rockets, but will require more rocket tests and more nuclear tests to make these militarily useful.
“In other words, North Korea does not yet have the ability to reach the US mainland with a nuclear-tipped ICBM – but they continue to work in that direction.”