 He was born in 1450 as the second son of Sejo of Joseon.His formal name was Prince Haeyang, but he was promoted to crown prince at the age of 8, after the sudden death of his elder brother, Crown Prince Uigyeong.In 1468, when he was 19, his father Sejo turned over the throne, but since he was not yet 20 years old and had been physically weak since his childhood, Queen Jeonghee, who was Yejong's mother, came to rule the nation instead.