אֲשֶׁ֨ר הָיָ֤ה דְבַר־יְהוָה֙ אֵלָ֔יו בִּימֵ֛י יֹאשִׁיָּ֥הוּ בֶן־אָמ֖וֹן מֶ֣לֶךְ יְהוּדָ֑ה בִּשְׁלֹשׁ־עֶשְׂרֵ֥ה שָׁנָ֖ה לְמָלְכֽוֹ׃
to whom the word of the LORD came in the days of Josiah the son of Amon, king of Judah, in the thirteenth year of his reign.
Ibn Ezra
Hath called me from the birth, I shall explain this in the commentary on Jer. 1:2.1Comp. c. xlviii. Note 10.
Rashi
twenty-three years Josiah reigned thirty-one years (II Kings 22:1). Subtract twelve years from them (Jer. 1:2). This leaves nineteen, and four of his son amount to twenty-three.