How old does a child have to be to decide who they live with

Depends on your location

ANY question on law needs to include your general locale as laws vary by area.

As far as I'm aware, nowhere does a CHILD get to make the decision. In all the US States that decision is made by a court. Until someone has reached majority (age 18 in most of the US), they cannot make the decision. In some states, the older a child is, the more weight a judge is required to give their preference.

In any case, if the child is under 18, then the non custodial parent has to go to court to request a modification of the custody order. If they are over 18 they can live wherever they want.