Asked if United have started thinking about replacements for Ferguson, Charlton told talkSPORT: "No, we've not, because it's just a waste of time.
"It could be next week [when he retires], it could be ten years, it doesn't matter. We will wait for him. We can't tell him what to do, he is fantastic.
"He loves the game and every time he thinks about retiring he has to look carefully at what decision he is making because it is a tough thing for him to do.
"He is so positive and knowledgeable about the game that he won't make any mistakes. If he retires, it will be his [decision]. He is the greatest thing that has ever happened to our club."