how old does a child have to be to decide which parent to live with?

Technically it's not her choice. If her biological father applies for custody then she is old enough for the courts to consider her opinion but she can't decide herself. For her to make the decision she'd have to be 18.

4

A judge would be making that decision, if it was disputed between the parents. A judge could give weight to the children's feelings at any age, but thirteen is certainly going to get more consideration than somebody younger.

But you would want to check the custody agreement. Since you already live in a different state, I doubt there's significantly more concern, at most you might be asked to pay more of the transportation costs.

My mom has a friend going through that. We're in CT and she's moving to AZ and has a 14 year old daughter. The father is fighting for her and though she wants to go with her mom, she can't decide till she's 16 years old according to the court.

12-years of age.

I believe in most states the age when they can start deciding is 14.

It also depends on if you have sole custody or not with her father.

Any age