Kim grew up with his older brother Kim Jong-chul under the 'watchful eye' of their father, Kim Jong-Il.
Kenji Fujimoto, head chef to the dictator, described how Kim Jong-un showed his famously ruthless temperament at a young age.
According to the Daily Star, Fujimoto said the two brothers enjoyed playing basketball as children and had their own court in the royal palaces.
He added that though the elder brother was a kind team captain, Kim would shout at the players.
Fujimoto revealed that he was sent abroad to buy exotic food such as fish from Japan or Big Macs from Peking for the two brothers.
Fujimoto said Kim learned driving at the age of seven, and got a bulletproof Mercedes-Benz.
Kim was described as very mature for his age, always playing the role of leader among his peers.
At 14 he began smoking and would lounge on one of his two personal yachts or played with his jet ski.
According to the report, Fujimoto revealed that at the age of 18 he was introduced to his father's 'pleasure brigade' - a team of teenage girls expected to strip, massage and perform sexual favours for him.
Fujimoto, who has a wife and child in Pyongyang, defected to Japan in 2001 after 10 years in the family.
Kim Jong-un forgave him for his "betrayal" but the former head chef is still terrified for his life.