I assume by "joint custody" you are referring to shared parental resonsibility. That smply means that you each have equal decision making authority fo rthe children and that each of you should work towards compromise regarding your child's best interests. The short answer is that there is no "legal age" in Florida for a child to make the determination of where he or she will live. On the other hand, many of the Courts take the common sense approach that an older teenager really cannot be forced to be with a parent. You will need to trust your attorney's analysis of this situation. The last order of the Court is usually enforceable by contemt unless you voluntarily allowed the situation to evolve to what it has become. The Court generally views factors such as the length of time the contact has changed,the reasons for requesting strict adherence at this time, if the request for enforcement has gone ignored, and whether either party has filed a supplemental petition for modification of the final judgment of dissolution of marriage. Also, you stated your ldest child is 16. If a modification is filed, these cases sometimes drag on for a year or more. The question then becomes whether it is worth it (financially, emotionally, and impact on quality of life) to argue over what amounts to 2 overnights each month? Think it through carefully, listen to the advice of the attorney you choose to hire, and make a sound decision based upon what is best for your children and yourself.
There is no magic age when children get to decide where to live. The best interest of the child is the standard.
The law regarding a child's 'right'* to choose is a matter for each State and jurisdiction. The judge in most States, not the child, makes the decision based on the best interest of the child. Although not a standard by any means, many States have begun to give 'consideration' to a child's declaration of custodial preference when the child reaches the age of twelve or thirteen, sometimes fourteen. There are even cases when children of age 9 are allowed to testify.
The judge is normally given almost unlimited latitude in whether or not she or he listens to a child and how much weight to give to the child's wishes. In short, there is no specific "age" but the younger the child the less likely for a judge to give the stated preference much weight.
Good luck to you.
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