The correct answer in the state of Iowa is that there is no magic number. There is no cutoff or minimum age for a child to decide which parent with whom he or she wants to live. Or, as a judge put it recently when a self-represented litigant asked this question of them during a hearing, "a child cannot choose until they are 18."
Iowa Code 598.41(3) provides a lengthy list of factors for the Court to consider when deciding what custody arrangement is appropriate. The factor in subsection (f) states that the Court should consider "[w]hether 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" when deciding what is in the child's best interests.
The case law expands upon that factor, giving different considerations for the Court in deciding how much weight to attribute to a given child's statement of where they wish to live. A child's preference is not controlling, but it is important. The Court considers the child's age and educational level, the strength of the preference, the intellectual and emotional makeup of the child, relationships with family members, the reasons for the decision, the advisability of recognizing the preference and the realization that the court cannot be aware of all the factors that influence the child's decision. See In re Marriage of Ellerbroek, 377 N.W.2d 257, 258-59 (Iowa Ct. App. 1985).
Each case is going to require specific analysis, which is more complicated than something you will find in a blog post on an attorney's website. If you are interested in establishing or modifying child custody based on your child's wishes, contact us to schedule your initial consultation. We would be happy to discuss these matters with you.