Steinbeck offers us the clearest picture of Lennie's lack of mental development in the opening section of Of Mice and Men, demonstrating that Lennie
is merely a child in a world of selfish adults.
We see this in Lennie's childish insistence on ketchup to go with the beans that George is cooking, in Lennie's request for a story before sleeping, in his fondness of soft things (mice) and in his obvious duplicity which George instantly penetrates (when Lennie retrieves the mouse across the stream and tries to hide it from George).
In addition to the fact that Lennie needs someone to take care of him, he is also shown to have a terrible and child-like memory. Lennie must be coaxed to remember the escape plan if anything goes wrong, as he must be coaxed into remembering what happened in Weed. He also can't clearly recall his Aunt Clara who raised him.
His plan to run off into the hills and live in a cave is also indicative of an underdeveloped mentality.