Cease to hear instruction, my son,
and you will stray from the words of knowledge.
Stop hearing instruction and you will not only stop learning more, but you will stray from the path of knowledge. Ceasing to learn more is not a matter of slowing down and letting others pass you by. Rather, it is straying off the path of knowledge altogether. A patient who stops receiving physical therapy too soon, not only stops improving but regresses. So it is for anyone who thinks he no longer needs to hear instruction.
James says that if anyone is a hearer of the word and not a doer, he will forget what he learned (James 1:22ff). Solomon is saying that if anyone thinks he no longer needs to hear, he will begin to do what is wrong. He will stray. He begins to interpret wrongly what he sees. He will make mistakes in judgment. And worse, he will lose his ability to discern when he makes mistakes. He no longer has the ability to hear.
The mark of wisdom is not reaching a plateau where you no longer need instruction. It is reaching the point where you delight in hearing instruction. Certainly for the believer, it is the delight in hearing instruction in the Word of God. The true person of knowledge cannot hear the gospel enough; he cannot receive enough instruction in the teachings of grace nor of the law which guides him in the way of righteousness; he cannot hear too many times about the greatness of his God and the wonders of his works; he cannot desire too much to live a life that glorifies his Lord.