“I am a young lad.” The statement, “I am a young lad,” while being true to a degree considering the culture, is in this context actually more of a hyperbole, an exaggeration, used to express how he felt. Solomon is one of the few kings in the history of Judah and Israel whose age when he began to reign is not given in Scripture (cp. 2 Sam. 2:10; 5:4; 1 Kings 14:21; 22:42; 2 Kings 14:1-2; 21:1, 19; 22:1; 23:31). The date Solomon came to the throne is debated. Some Jewish scholars say around 12 years old, and Josephus says at 14 (Antiquities; Book 8, chap. 7, section 8; but in the same sentence Josephus contradicts Scripture and says Solomon reigned 80 years and died at 94, ct. 1 Kings 11:42), but Solomon’s actual age was likely closer to 20 (Hastings Bible Dictionary).
There are a number of reasons to believe that Solomon was about 20 or perhaps even a little older when he took the throne. He had already had at least one son (Rehoboam) by the Ammonite woman Naamah before he became king (1 Kings 14:21). Also, in his first year as king he showed unusual maturity and decisiveness in the way he handled men who had become his adversaries. He executed his older brother Adonijah and also Joab the captain of David’s army, and he removed Abiathar from being the high priest (1 Kings 2:25, 27, 34). Also, there is no indication that he had any advisors or mentors help him run the kingdom, even from the very start.
Furthermore, Scripture says he reigned 40 years, but if he was only 12 or so when he started reigning as king, then he would have died at 52 or so. However, Scripture says, “when Solomon was old that his wives turned away his heart after other gods” (1 Kings 11:4). However, it seems clear that it took a number of years for Solomon’s heart to turn to other gods and for him to have time to build temples to Moloch, Chemosh, and other gods and establish sacrificial practices for them, which would involve the gathering and participation of at least some pagan priests (1 Kings 11:7-8). To be considered “old” in that culture it would seem Solomon would have had to have been at least 50.
“I don’t know how to go out or come in.” The phrase “I don’t know how to go out or come in” is the figure of speech polarmerismos. Polarmerismos occurs when two extremes are used to represent a whole. Here, “go out and come in” represents the two extremes of life, when a person goes out in the morning and comes back in at night. Thus, in this context, the idiom means, “live life.” Solomon could have said, “I am but a young lad, I don’t know how to live life as a king,” or, “I am too inexperienced to be king,” but he used the idiom.