What age does your child have to be to choose if they want to live her mom or her dad?

A child can NEVER choose. At least not in the US. Until the child reaches the ago of majority (usually 18) and are no longer a child. They can't choose. The decision is up to a judge.

Some areas (ANY question on law needs to include your general locale as laws vary by area.) do have guidelines on how much weight to put on the child's preference, but the decision is still the court's.