Albert Barnes Commentary 1 Kings 11:4

"For it came to pass, when Solomon was old, that his wives turned away his heart after other gods; and his heart was not perfect with Jehovah his God, as was the heart of David his father." — 1 Kings 11:4 (ASV)

About fifty or fifty-five. Based on his age when he took the throne (see note on 1 Kings 2:2), he could not have been more than about sixty when he died.

The true nature of Solomon’s idolatry was neither a complete apostasy from which there could be no recovery, nor a mere toleration that was more praiseworthy than blameworthy.

Solomon never openly or wholly apostatized. He continued to attend the worship of Yahweh and faithfully made his offerings three times a year in the temple (1 Kings 9:25). However, his heart was not “perfect” with God. The religious earnestness of his younger days was weakened by wealth, luxury, and sensuality. This decline led to an increasing worldliness, which resulted in worldly policies and a religious broad-mindedness that arose from his contact with many different human opinions.

His lapse into deadly sin was undoubtedly gradual. Partly from ostentation and partly from the sensuality common to Eastern monarchs, he established a harem on a grand and extraordinary scale. To gratify these “strange women”—that is, foreigners, who were admitted for political reasons or for the sake of variety—he built magnificent temples to their false gods directly opposite Jerusalem, as obvious rivals to the Lord’s temple.

He thus became the author of a syncretism that sought to blend the worship of Yahweh with the worship of idols—a syncretism that held a fatal attraction for the Jewish nation.

Finally, he himself appears to have frequented the idol temples (1 Kings 11:5, 10) and to have taken part in the dreadful impurities that constituted the worst horror of these idolatrous systems. In this way, he was apostatizing in practice, even though he theoretically never ceased to believe that Yahweh was the true God.