At what age did Jehoiachin begin to reign as king?
2Kgs 24:8 (AKJV/PCE)
(24:8) ¶ 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.
2Chr 36:9 (AKJV/PCE)
(36:9) ¶ 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.
Jehoiachin was 8 years old when he began to reign as co-regent, and 18 years old when he began to reign alone.
The practice of the reigning king appointing a co-regent for various reasons is not unprecedented, examples being found in 1 Kings 1:33-40 and 2 Chronicles 26:21.
While we have no specific scriptural reference explicitly saying so, such as the examples given, it is a plausible explanation, especially considering the tumultuous and uncertain political environment at that time; and can be gleaned from the scripture itself, by recognising the bolded phrase (below) as the fixed reference point, or constant, and the variable (ages) as indicating the factual existence of that period of co-regency of 10 years.
2Kgs 24:8 (AKJV/PCE)
(24:8) ¶ 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.
2Chr 36:9 (AKJV/PCE)
(36:9) ¶ 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.
There is no contradiction or error.