According to the Chrónica latina de los reyes de Castilla, he was 16 years old when he became king of Castile in 1217 which would mean that he was born in 1201. Theo Chrónica latina de los reyes de Castilla ông được 16 tuổi khi ông trở thành vua Castile năm 1217 có nghĩa là ông sinh năm 1201.