[2 Kings 24:8 KJV]:
"Jehoiachin was eighteen years old when he began to reign, and he reigned in Jerusalem three months. And his mother's name was Nehushta, the daughter of Elnathan of Jerusalem."
[2 Chron 36:9 KJV]:
"Jehoiachin was eight years old when he began to reign, and he reigned three months and ten days in Jerusalem: and he did that which was evil in the sight of the LORD."
The more recent NIV has it differently based on manuscript evidence and an examination of that evidence on a case by case basis:
[2 Chron 36:9 NIV]:
"Jehoiachin was eighteen years old when he became King, and he reigned in Jerusalem three months and ten days, he did evil in the eyes of the Lord."
Manuscript evidence when correctly examined ON A CASE BY CASE BASIS and not on the basis of favoring the majority of one type manuscript over another due to the false presupposition that one type is always more reliable than another no matter what, will provide the best rendering of what the original text was. This process must be accompanied by the use of normative rules of language, (in this case the original Hebrew), and the rest of the H.I.C.E.E method of interpretation which is the self-evident method to be used with the words of God's Word:
This procedure for interpreting/translating the words of God's Word from available manuscript evidence and from the best rendering into English leads in this case to the conclusion that Jehoiachin was 18 and not 8 so the correct rendering for 2 Chr 36:9 is 18 as rendered in the more recent NIV version. Manuscript evidence supports this: one Hebrew manuscript, a number of Septuagint manuscripts and Syriac show 18 years.
Consider this succinct statement re: this matter from THE NEW SCOFIELD STUDY BIBLE, NIV, Editor: C. I. Scofield, Oxford Univ. Press, 1984, pp. 414-5:
[size=+2]"In copying manuscripts, mistakes in numbers sometimes occur. Many disagreements between numbers in Samuel and Kings, and those in Chronicles, are alleged. Actually, out of the approximately 150 instances of parallel numbers in these books, fewer than one-sixth disagree. In two cases a different number is given for the age of a king at his accession (cp. 2 Chr. 22:2... with 2 Ki 8:26 and 2 Chr 36:9... with 2 Ki 24:8); in the other thirteen cases of this type, numbers agree. Certain disagreements are very small (cp. 1 Chr 21:5, as to Judah, with 2 Sam 24:9; 2 Chr 2:2, 17-18 with 1 Ki 5:15-16; and 2 Chr 8:18 with 1 Ki 9:28). Sometimes the apparent discrepancy disappears on careful study (cp. 1 Chr 21:25 with 2 Sam 24:24; 2 Chr 3:4 with 1 Ki 6:2). [/size]