Jamieson, Fausset and Brown on 1 Kings 11:4

4. when Solomon was old--He could not have been more than fifty.

his wives turned away his heart after other gods--Some, considering the lapse of Solomon into idolatry as a thing incredible, regard him as merely humoring his wives in the practice of their superstition; and, in countenancing their respective rites by his presence, as giving only an outward homage--a sensible worship, in which neither his understanding nor his heart was engaged. The apology only makes matters worse, as it implies an adding of hypocrisy and contempt of God to an open breach of His law. There seems no possibility of explaining the language of the sacred historian, but as intimating that Solomon became an actual and open idolater, worshipping images of wood or stone in sight of the very temple which, in early life, he had erected to the true God. Hence that part of Olivet was called the high place of Tophet (Jr 7:30-34), and the hill is still known as the Mount of Offense, of the Mount of Corruption (2Ki 23:13).