A 13 year old could plan where they want to live but they can't decide and suddenly leave their parents house and move in with a friend. That would happen when they are older like 18 or 16.
No, since you are legally not an adult, you will need consent from your parents or legal guardian to move out of their house
Yes, so long as the grandparent is able to care for the child and he/she has the permission or rights to take care of them as a guardian.
no
twelve because if they are twelve they are twelve.