Why does Charley say Willy hasn't grown up?
Charley has a love/hate relationship with Willy. Although the two neighbors bicker about things, Charley evidently sincerely cares for Willy. He tries to get him to stop obsessing about Biff, to be honest with himself about his career, and to take the job that he is offering him. Willy, who is in a precarious mental state during the play, insults Charley, rejects his advice, rejects his job offer while accepting his handouts, and swears at him. Charley asks, "When the hell are you going to grow up?" He feels Willy is clinging to the past, such as his relationship with his current employer's father, when Willy should be able to change with the changing times. Charley also asks him, "Why must everybody like you?" This indicates that Willy is stuck in an immature mindset of needing approval. He should have come to terms with who he is, his skills and his shortcomings, long ago. But because Willy cannot face the truth, he remains immature, still struggling with issues of identity and self-worth even though he is nearing retirement age. Willy stands in stark contrast to Charley's son, Bernard. Willy has always mocked and belittled Bernard, and now Bernard has an excellent career as a lawyer, has a family, and moves in impressive social circles. He is even arguing a case before the Supreme Court. When Willy asks Charley why Bernard didn't mention the Supreme Court case to him, Charley says, "He don't have to--he's gonna do it." Willy, on the other hand, is constantly bragging about things he is going to do or lying about things he has done. This also shows Willy's immaturity. Charley, because he truly cares for Willy, points out to him his need to grow up. Unfortunately, Willy ignores Charley's attempted intervention.
Willy hangs on to petty fueds with Charley. His pride is childish in this context. He wants to give Charley advice and bicker with him while accepting money from him on the side (because he, Willy, is failing to make sales).
Willy has never really settled understand his and his son's limitations. He does not hold Charley in any high regard, yet Charley is more successful as a businessman and a father than Willy is.