At what age can a child choose to live with whichever parent? Is there a law in the st. of Florida on this?

At age 18.

Nowhere in the US is a minor child (under 18) allowed to choose what parent they will live with. In most states a Family court judge will listen to the child's preference, but the judge makes the decision based on what they consider best for the child.

So, one parent needs to apply for a change in the custody order and the child can then express their preference to the judge.

When the child reached the age of 18. They can move in with which ever parent they want (as long as that parent will allow it) or on their own if they wish.