I am almost 15 I want to move in with my grandmother who loves me and wants to take care of me My mother and step-dad don't treat me right How old do I have to be to go live with my grandmother?

If you are being mistreated by your mother and step father you can take it up with child welfare especial y if you have a grandmother who is willing to accept responsibility for you. however make sure that you treat this situation seriously and truthfully because whatever action you chose to take there are always consequences.