Lennie is a grown man, but he talks like a child because of his mental handicap.
You can tell by the conversations between Lennie and George that Lennie depends completely on George. Lennie looks up to George as a big brother or father figure. He does not really think about anything except the present.
Lennie spends a lot of time talking about rabbits, mice, dogs, and things he can pet. He is obsessed with soft things. His memory is not very good, so he often asks George to repeat things to him. He even picks up a dead mouse in the first chapter.
"Uh-uh. Jus' a dead mouse, George. I didn't kill it. Honest! I found it. I found it dead." (Ch. 1)
The interaction between George and Lennie in the first chapter introduces the reader to the unequal relationship between the two of them. George tells Lennie he thinks they should not drink the water, while Lennie tells George happily that the water is good, not paying any attention to whether or not it might be running. Then Lennie finds a mouse and tries to pet it, swearing he did not kill it.
Lennie’s childlike innocence really comes through in how he talks. Another incident is when Curley picks a fight with him. Lennie is scared, and does not know what to do.
Lennie gave a cry of terror. Blood welled from his nose. "George," he cried. "Make 'um let me alone, George." He backed until he was against the wall, and Curley followed, slugging him in the face. (Ch. 3)
Even though Lennie is bigger and stronger than Curley, he is genuinely scared. He does not know what to do in this situation, when Curley is picking a fight with him. George has told him to stay away from Curley, and Lennie also does not pick fights. He relies on George to protect him, because he is in many ways still a child.
When George calls out to Lennie to get Curley, Lennie crushes his hand, so that Curley will stop hitting Lennie. George knows that this is the beginning of the end for them. He cannot protect the gentle giant for long now.
Lennie is an adult man with the mental capacity of a child. The character of Lennie can not be separated from his best friend, George. George is basically the mouth-piece for Lennie and helps him navigate through his life.
The way Lennie speaks is very babyish, often repeating phrases that he has learned to memorize, but does not have his own thoughts or opinions on matters. He is mentally incapable, and for those reasons relies on George to be his spokesperson.
There relationship is one of dependence. Lennie would not be in existence if not for George. George shows frustration and impatience with Lennie, but at the same time,has a sense of responsibility when it comes to Lennie, almost as a parent would have to a child.
In summary, Lennie can not be clearly characterized without mentioning George, because Lennie would not be were it not for George. So the way Lennie speaks and the things he says is a result of what he has been told over and over again by George.
Lennie has a mental disorder which causes him to speak in a child-like way even though he is an adult. You can see throughout the book that Lennie is very curious, which often causes him to get into trouble, but he never understands why. He has a child's mindset; he is innocent and doesn't have ill intentions. He is dependent on George to help him remember things and understand situations. In other words, Lennie speaks in simple terms, often repeating himself, and with innocence.