Of Mice and Men- Unanswered Questions

So. I just started reading Of Mice and Men. I am at the part where George and Lenny start a new job. George is covering for Lenny claiming that they are cousins and he had a brain injury as a child that made him dumb. It seems like Lenny has special needs. What is wrong with him? How did it happen and was he born with a cognitive disability or did something make him that way. Since we know they are not related, what interest does George have in protecting him? Why does he care so much for someone that he is not related to or in a relationship with? What time in history does this story take place?