How old was Hezekiah when God added 15 years to his life?

Hezekiah was the King of Judah from 745-717 B.C.E.

He became King when he was 25 years old. So he was born in 770'ish.

(2 Chronicles 29:1) 29 Hez·e·ki′ah himself became king at the age of twenty-five years. . .

In 732 he was granted 15 more years of his life. He was around 38 years old.