For in the eighth year of his reign, while he was still young, he began to seek the God of his father David; and in the twelfth year he began to purge Judah and Jerusalem of the high places, the wooden images, the carved images, and the molded images. 2 Chr 34:3
Josiah became king when he was 8 yrs old. By the 8th year of his reign, he would have been 16. Hence, based on what we are told in this verse, he sought the Lord when he was 16 yrs old. And by the 12th year of his reign (when he would have been 20 yrs old), he began to purge Israel of its idols.
God gives grace to some when they are young such that they have a longer run way for ministry, just like Spurgeon. Others turn to Christ when they are much older. Whether one turns to Christ early or late in their lives, let God be glorified for it is Him who gives grace and His Spirit to work in these hearts. We should continue to fervently pray that our children may turn to Christ early. May God have mercy upon them.