וַיִּקָּבְצ֣וּ עָלָ֗יו אֲנָשִׁ֤ים רֵקִים֙ בְּנֵ֣י בְלִיַּ֔עַל וַיִּֽתְאַמְּצ֖וּ עַל־רְחַבְעָ֣ם בֶּן־שְׁלֹמֹ֑ה וּרְחַבְעָ֗ם הָ֤יָה נַ֙עַר֙ וְרַךְ־לֵבָ֔ב וְלֹ֥א הִתְחַזַּ֖ק לִפְנֵיהֶֽם׃
And there were gathered unto him vain men, base fellows that strengthened themselves against Rehoboam the son of Solomon, when Rehoboam was young and faint-hearted, and could not withstand them.
Rashi on II Chronicles
and they overwhelmed Rehoboam the son of Solomon and Rehoboam was young and soft-hearted and since they found in him humility and soft- heartedness, they overwhelmed him.
Rashi on II Chronicles
young - Heb. נַעַר. Although he was forty-one years old when he reigned, he is called a youth (נַעַר), as I explained above (I Chron. 22:5) in regards to Solomon.