I have also looked into this. In missouri there isnt really a certain age but the courts do take into consideration the childs opinion if they seem mature and intelligent enough. they look at both homes also and there has to be a good reason for a change in custody. However, you do need less of a reason once the child is older as described above and can speak for themselves and less likely to be "coached" by any one parent. A good place to start is with advice from a lawyer and you might consider initiating a meeting for your son and yourself and mother for counseling. The counselor could get to the root of the problem and help you guys better parent the child. Also it could be your son is the problem and is manipulating you based on your dislike of the other parent. She may just be discipling him and it may but what he needs and you may be the easier of the two homes for him to get away with what he wants. He is a teenage boy. You have to be prepared to look at all angles because it is certain to get very costly for you and a guardian ad litem and the courts will be looking into any and all things. Either way, good luck to you!