Here it says that he was 18 when he became king. In Divrei ha'Yamim II, 36:9 it says that he was eight, and he reigned for three months and 10 days in Yerushalayim!
Radak: Perhaps Yehoyakim made him king in the first year that he himself reigned 1 , so that Yehoyachin would reign after Yehoyakim dies. Perhaps he had other sons [and he wanted Yehoyachin to succeed him, even though he was not the oldest]. Also, his kingship depended on others. He did not know if he will be removed from his kingship, like his brother was. Yehoyachin was eight at the time. When his father died he was 18, and he ruled for three months. The verse here did not bother to mention the other 10 days.
Radak citing Seder Olam 25: It was the 18th year of Nebuchadnetzar, and eight years from when exile was decreed.
Malbim citing Mahari: Divrei ha'Yamim means when he was eight years, three months and 10 "Yamim", i.e. years (i.e. 18 years and three months) 2 .
Malbim: When Pharaoh Necho deposed Yeho'achaz and made Yehoyakim king, he knew that the people had chosen Yeho'achaz, and when Yehoyakim will die, they would want to make Yeho'achaz' sons king. Therefore he made also Yehoyachin king, that he will succeed his father, so Yeho'achaz' sons would not claim kingship after their father dies. Yehoyachin was eight at the time.
