וַיִּמְלֹ֨ךְ שְׁלֹמֹ֧ה בִֽירוּשָׁלִַ֛ם עַל־כָּל־יִשְׂרָאֵ֖ל אַרְבָּעִ֥ים שָׁנָֽה׃
און שלמה האָט געקיניגט אין ירושָלַיִם איבער גאַנץ ישׂראל פֿערציק יאָר.
Rashi
And King Rehoboam strengthened himself in Jerusalem And he explains why, because when he was first made king, he was forty-one years old; therefore, he had the strength and the power to strengthen himself. He was a year old when David died, for Solomon reigned forty years, and Solomon had not yet reigned when Rehoboam was born. We find that Rehoboam was forty-one years old when he reigned.