Iowa law that is relevant to custody is here. At no point is a child prohibited from expressing his wishes to a judge. §598.41 prescribes how the court determine custody. Para 3(f) specifically includes
Whether the custody arrangement is in accord with the child’s wishes
or whether the child has strong opposition, taking into consideration
the child’s age and maturity.
This would have been the case at the initial custody hearing, and any petition to modify custody. There is no rule setting a lowest age for considering a child's wishes, but there may be a tendency to consider "teenager" as potentially old enough.
A custody arrangement can be modified, but the petitioning parent has to show that there has been a “substantial and material change in circumstances” since the initial decree.
But let's assume that the parents have joint custody, then none of that applies and you live where your parents agree you should live. You don;t have legal standing to petition for a change of custody, but you might (if there is a good enough reason) succeed on a petition for emancipation when you are 16.