Charles Spurgeon Commentary 2 Chronicles 28:1

"Ahaz was twenty years old when he began to reign; and he reigned sixteen years in Jerusalem: and he did not that which was right in the eyes of Jehovah, like David his father;" — 2 Chronicles 28:1 (ASV)

Ahaz was twenty years old when he began to reign, and he reigned sixteen years in Jerusalem:

So that he died before he reached the prime of life; he was cut off by God in the very midst of his sin.