I'm afraid men are not always quite as clever as they think they are. You will learn that when you get a bit older, my girl.
In Roald Dahl's "Matilda," the quote reflects a common theme regarding the underestimation of wisdom and intelligence in others, particularly in men. It expresses a sense of caution and a hint of irony about how people often overestimate their own cleverness. The speaker, through this observation, emphasizes the importance of humility and self-awareness in one's abilities.
The advice given to the young girl suggests that as she matures, she will gain a clearer perspective on human nature and the flaws that accompany arrogance. It implies that true intelligence lies in recognizing one's limitations and learning from experience, rather than boasting about perceived cleverness.